Can't speak specifically on the Ace 300 but the VDI on the ACE 250 is decent on targets upto 6" were iron is not a factor. So for example if you bury a penny, dime, quarter and there are no nails or pop tabs close by it will tell you what your digging. Of course there is a whole bunch of other items you will be fouled by that will show up as well. The Compadre is just a beep and dig machine but it has a reputation and I have seen it first hand of finding small gold on the tot lots and parks. You'll be digging trash for sure but you will learn alot.
I would say you will get more depth on the 300 and for $250 it's probably the most recommended vdi machine on the market. I choose to buy the Fishers F22 at $229 and there are some features I really like about the F22. I love the all metal and jewelry mode, but I have had issue's with the cheap buttons and recovery speed & unmasking is terrible. FYI the liberator looks like it has a similiar screen and PP button?
I got hung up on a pinpoint button when I was first looking but with a good machine you will almost never use it to locate a target. When a tesoro beeps you know exactly were to dig.
Were I live you can probably sell an ACE quicker because it seem like every weekend warrior is using them.