Tennisplayer is much beefier than Tennisone and the individual pieces are, on average, of a much higher quality. I've toured the sight at my friend's place, who is a member, and was very impressed with the layout and detail.
I'd definitely join at a slightly lower monthly price where I was locked in for a year of monthly renewals (it's the "under-the-bridge" theory: I'll spend more in small increments. Look at the credit card industry: if you want to boil somebody, you have to turn up the heat slowly). The current rate is fine, but I think there are a lot of people that would get hooked with a more attractive offer, i.e., the sight is that good; if you spend time there, you'll want to go back. Moreover, you can tell that it will have a nice influx of new content. I realize the creator(s) need to recoup their costs, so they might not have much price flexibility. If my pimp lets me keep more bling from my tricks, I'm gonna jump in. Overall, though, best tennis sight on the internet hands down; many props! That slow mo stuff is incredible.