I think if you're mostly into tactical RPGs and RPGs, PS3 is probably your best bet. Stuff like Valkyria Chronicles, Cross Edge and White Knight Chronicles (to name a few) are PS3 exclusive. Also the range of RPGs on PSN is better than XBLA, with Vagrant Story just being made available and Square Enix looking like they'll release more of their back catalogue through PSN.

Like you say, the biggest titles tend to get a multi-platform release these days so you won't miss out on too much. While I prefer my Xbox 360 to the PS3, I think with the tastes you've described, PS3 is probably better for you.

I would like to point out that I am not a raving lunatic fanboy for either platform!