Just to throw another seller in the mix, java-plants.com just won best online retailer in October's issue of PFK magazine (Yay, my first copy!) I've heard good things but never used them. Might order some stuff from them and see how it goes
I've used Greenline and have been pretty impressed. I think if I had to criticise, it'd be that there website needs updating. Its a bit awkward to navigate and some of the scientific names are slightly wrong on some plants (I tend to browse and then google plants I like to get more in-depth information). And theres sometimes scarce information about some plants.
That being said, it only takes a little research to figure out the info and I don't really see it as a big problem, and would happily recommend them to anyone