I bought my "dream 'bent" as my first one. Unfortunately I didn't take it for a long enough test ride so I didn't know it gave me recumbutt; I ended up selling that one (losing on the resale) and bought my current one which is great for me. So I definitely recommend a long test ride, at least 30 minutes preferably an hour, of any 'bent you might buy. Aside from that, I agree with the other posts about going for one you can grow into.
The only thing I can think of the shop owner might be getting at is it's harder to get used to starting and stopping on some higher performance 'bents like highracers with high bottom brackets. But that's something you'd pick up in days, not months. What can take months is your muscles getting used to the subtle differences between pedalling a 'bent and a DF. But you have to get used to that on any type of 'bent.