ScreenSelect and LOVEFiLM have the best sites by far. Some of the less professional outfits have really horrible-looking web sites with white-on-black text, or pages that appear to have nothing on them (presumably because they didn't test them on non-Windows computers), or links to third-party sites slap in the middle of the page.
Even worse are the ones which look professional but have broken search engines: type in a director's name and it tells you "we have no films by this director", but type in the film titles one by one and it miraculously finRAB them. One very smart looking site has half-a-dozen or so films by Francois Truffaut, except that half of them have been catalogued under the name Francois Truffant.
As far as making sure you get your top preference, I don't know the answer. I'm still on the LOVEFiLM trial and I've had no problems, although films don't always get checked back in in the order I've posted them. Royal Mail incompetence, or something in their systems holding things up? Presumably the volume of films returned is much higher after the weekend and lower before it, which would also affect what stock is available to send out. There must be a way to "play the system" but if you want to watch a specific film on a specific day online rental isn't the best way to do it.