ihuntrelics told the story real well. It really does depend on the kind of ground you're going to be hunting in, how much weight you can swing for hours at a time and what your budget is. I would strongly recommend that if this would be a first machine, not to get a TID machine as you'll lean too heavily on what the machine is telling you and you'll be walking over good targets. I've been doing this for a few years now and I use a Target ID (TID) machine to try and locate sites and when I locate a hot spot I'll go over to a non-TID and start digging pretty much everything.
Like ihuntrelics said, it largely depends on your needs. If I was just starting out again I would get a Tesoro Vaquero, as it will give the best pinpoint, will get targets at the 8"-9" average depth of where relics are found, in reasonable ground and its real lightweight and won't bust the budget. I need to add one of those back into my arsenal as I used to have an Eldorado (just like it, only a little earlier vintage) and like an idiot traded it because I hadn't been using it much while I experimented with other machines. The Eldorado is a great relic hunter as it will handle slightly bad ground better than almost anything I've used in the Tesoro lineup. Its too bad they don't make that model anymore.