I own a Garrett Infinium which is the big brother of the Sea Hunter Mark II. I can't be more pleased with the quality of the machine. I have also owned a White's PI and I still own Tesoro Land Machines. They are all good machines. I believe it comes down to personal preferences of the finer minutia related to varied machines.
The positives of the Garrett are that you can change coils on the spot at the beach. I don't believe that you can do that with the other two. The Garrett also allows you to switch between land headphones (much more comfortable when hunting the surf without getting your head wet), and water headphones for the obvious times when the surf is high or you decided to scuba or snorkel. The Garrett also has a discrimination capability. In the case of the Sea Hunter you get two modes of discrimination one of those is adjustable like any land vlf detector. I believe the Tesoro has that also, not sure about the White's ID although I know it does have that ID feature which is in the same ball park.
I don't believe you would be disappointed by either of the three. Once you get in the water and start finding stuff, you fall in love with whatever is in your hand. (Hopefully it is a detector!!!!!) YUK YUK
I would just say to read the reviews written by actual users. I believe you will find that many posts in other areas of the internet have some bias writings from dealers, etc. You can usually pick them out quickly. No detector is perfect so if you find that type of post, your either dealing with a dealer or someone who is in denial. They all have something that you can gripe about, just some much less than others. If you read enough of the posts of true users, you can get a pretty good idea if the machine has the features that you are looking for.
I hope you get what your looking for! The other advice given by MoTwister is true also. You can get a good deal on EBAY. Just make sure you buy from someone with good feedback and make sure that you ask all the questions you want about condition, whether it is their personal detector, how old it is, how many times used, what accessories come with it, will they throw in any accessories not mentioned like a scoop they have laying around, a pouch, a digger, etc. (I get some freebies this way regularly). I got my Infinium, almost new, with two extra coils and other accessories for 725.00 and free shipping! That was about 700 dollars cheaper than I could have bought it all from the wholesale dealers!! It is almost a year old and no problems so far other than sending it back to Garrett to get a computer update on the chips inside for 65 bucks.
Garrett and Tesoro have great service departments and great customer service. My White's lasted me forever without ever having to try the service folks.
Good Luck!!!
Jim