Following from what Melvin_Koh says, I'd suggest the difference between 2x2 and 3x3 AP is likely to be small unless all your clients support 3 spatial streams. This is unlikely.
You may also want to consider what the WAN bandwidth is - if you're plugging the RAP into an ADSL line, for example, Wi-Fi throughput isn't going to be an issue.
It'd suggest that for normal office work, web and e-mail 1 Mbps per user should provide a reasonable service level, and on those grounds a 303H would be fine.