Get what you can afford---then get out and use it. You will be fine. I started out by building a detector and I think any of the new ones made are great
Never used the 350 but have used Garrett detectors since '86...if you choose Garrett you will love their customer service, I bought a new detector once and after a month I hated it, after one email Garrett offered to take it back and gave me a choice of any same price detector to replace it!
Someone told me that an AT Pro, when used in the "standard (non-pro) mode" is the same circuitry as an Ace 350...can anyone verify this?