Should be a woman you're really attracted to. If from her ad she sounds wild, no holds barred, accept that this might be a bit of oversell, and feel confident that YOU'LL be the one setting the pace. I'm not sure EXACTLY what you mean by "conservative," but I'd never advise selecting a woman who strikes you as not very dynamic. If you get a mousy, quiet impression from her pics, it probably means she didn't spend much time or money on them. Maybe not much creativity either. More than getting a tender GFE from such, you'll stand a better chance of getting a mechanical session or even a bait & switch. You'll maybe leave feeling ripped off.
On the other hand, until you are comfortable reading the ads and reviews and "decoding" some of them, I'd stay away from the really high-end ladies, simply because you don't want to risk that much money. It's always buying a pig in a poke because even if some guys think a provider is a once in a lifetime wonder, she might simply not be your type.
Find someone you find beautiful. Discover by looking at the ads what type of woman you want. This isn't like high school. You CAN have the hottest lady, you CAN touch the sexiest body. Her personality? Well, that's more of a mystery, but MOST are terrific people. MOST are fun to be with, safe, and in their own way they really care about your happiness for the time they're with you. You don't have to "settle" for anything less than the best you can find.
Some day down the road you might want to do a pity session, taking your business to the Plain Jane who you think must not get many calls, but that's a LONG ways down the line. For the first encounter, go for the gold and give it your all! You deserve a good time!