These folks will have better information then I will. However being a total noob myself, I have been looking for comparisons. Here is a review, from another forum and not written by me. For the record I purchased an Ace 250 and a Tesoro Tiger Shark. Read as much as you can, then make up your own mind as to what you think will work best for you.
This last summer I bought an Ace 250 for the kids to use and as a loaner machine in case someone wanted to try their hand at detecting. After I used it a few times I found myself wishing I had gotten a Silver uMax instead. I was used to the single tone audio of my Tiger Shark and found the multiple tones of the Ace distracting. However, after using the ace a bit more I began to like it and I went and got a 4.5" coil for it. Then I got a good deal on a used Silver and got it and a 5.75" coil for it after having used the 4.5" on the ace. That sniper coil is fun, fun, fun to use and now it's permanently attached to the ace. I have used them both about the same amount of time now and I thought I would do a comparison for those thinking of these two machines.
Depth: The depth seems to be about the same for each with the stock coil, 6-7" on a quarter in an air test. This is with the sens. backed off a bit from max. I can't say how deep they will go in the ground as the deepest coins I have found are around 4". This is deep for my area as most coins are between 1" and 3". All of my old silver coin finds have been in this range also so I feel confident that most coins are shallow in my area. Most of the areas I hunt have undisturbed ground so most finds are shallow. The Silver gives a nice signal all the way out to the six inch range and then drops off suddenly after 7 inches. The audio gradually grows quieter as the target is moved farther out so that with a little practice you can usually tell the depth of your target with the Silver fairly accurately. The Ace gives the same tone to near its max depth with no drop off so you have to rely on the depth gauge wich is not always accurate. Also when you press the pinpoint button to detune for more accuracy in pinpointing the depth meter changes. At max depth the ace's tone will not be repeatable like the Silver's so its hard to tell if you have a deep target or just a piece of junk. Both detectors will hit deeper on larger objects like clock sprockets and larger coins.
weight: Both detectors are light and easy to swing but I give a slight edge to the Silver.
Audio: The Silver has a single tone. The ACE has a low tone for iron, a middle tone for foil through zn. cent. and a bell tone for coins. Even though the Silver has one tone it really talks to you with chirps and clicks and crackles that all have their own meanings that you must learn. To me the Ace doen't "talk" as much.
Discrimination: This is where the Ace shines. It has digital segmented notching which is a really nice feature. you can notch out everything but quarters without losing depth. On the Silver you lose considerable depth when you turn the disc. knob to max. With the silver I usually hunt in min. Disc. and thumb the disc. knob to identify targets for that very reason. There is no need to do this with the ace. You can set the notches any way you want. You can only accept the mid and low ranges if you are a serious gold ring hunter. Or set it for copper pennies and up and snarf up the coins. I like using the Ace in football fields and set it to only find quarters and dimes.
The discrimination on the Silver is excellent. Instead of having numbers written around the disc. knob it has words which is nice. Iron, Foil, 5 cents, tab, zn. cent, Max. At least on my machine these are dead accurate. Only quarters are repeatable at max. targets are knocked out cleanly at each setting. Unlike my other detectors crown type bottle tops disc. out lower so they are easy to spot. On the Ace you will get a bell tone (coin) for them but there is a trick. If you swing the coil real fast they will break up where a coin will stay repeatable. Any more I usually hunt in min disc. and dig everything as my good finds have gone way up since I have been doing this.
Sensitivity: I can run the Silver at the max boost range most of the time and it is quiet. I usually run the Ace one or two notches below Max. With the Silver set at max boost (the red zone on the know) I gain about 2" of depth. I get some falsing at maximum on the Ace.
Target I.D.: You would think that the Ace would have this one hands down being a TID machine but this is not so, in my experience. By thumbing the disc. knob on the Silver I believe I can tell what the target is 70% of the time. The Meter on the ace is a little less accurate. It will sometimes jump around between penny, dime, quarter and 50 cents. Sometimes it will say penny when its a dime and quarter when its a dime. When it locks on to one segment though it is usually accurate. I can tell the difference between a zn. cent, copper penny, dime and quarter with suprising accuracy on the silver by where they drop off on the scale and the disc. knob is conveniently located by your thumb on the Silver. If I just wanted to find gold jewelry, or only quarters then the Ace would have an advantage with its digital notch. Although I can easily cherry pick with both. On the Silver its just a matter of thumbing the disc. I give the advantage to the Ace because with the notch feature you can set it for mid range targets and not hear the others whereas with the Silver you hear all targets.
Pinpointing: Pinpointing is excellent with both detectors. Only the Ace has a dedicated pinpoint mode which is VCO. I have no trouble at all pinpointing the Silver in Disc. mode.
Controls: The Silver is about as simple as it could be with only a discrimination and sensitivity knob to mess with. Even though The Ace has more features it is also very easy to use. I really like the digital notch system and the different customizable programs. I have the "custom" mode on the ace set to accept foil, nickels and tabs for when I want to only hunt for gold jewelry.
Coils: The Silver uMax has a much greater selection of coils with a 4 incher all the way up to 3" x 18" widescan. The Ace does have the 4.5" Sniper coil which I highly recommend. I haven't checked into larger coils for the Ace.
Conclusion: Both detectors are excellent choices for the money with the Ace being a little less than the Silver. You definitely get more features with the Ace. If I had to pick one over the other I would choose the Silver as I really like Tesoros and I like a simple beep and dig machine. I usually loan the ace out to beginners with the Sniper coil becaue it is easy to pinpoint and fun to use. I hope this might help some who are trying to decide between the two. You really can't go wrong with either.