Go with .151 if you can - it's fast and very stable. If not, the official .148 is still way better than .75.
I agree with Bundy. If you feel comfortable doing it, I'd highly recommend doing an upgrade to 151. It's quite a bit better (memory management, animated transitions, etc) than even the official 148.
If you don't feel comfortable installing a non-Verizon OS, upgrade to 148. It's well worth the 20 minutes... just don't stay with .75
.148 is a solid build though there is an awful color banding issue. If you're comfortable, .151 is MUCH better but I recommend reading through the forums to learn exactly what to do...
When it comes to performance 148 or 151 is better that 75.
Now as for color banding, ask a friend who has 148 or any other os on their storm to see for your self. And when you decide to change your os look at my sig.
What ever you do...I recommend upgrading from .75.
The easiest thing would be to upgrade to .148, since it's "official" from verizon; and if this is your first rodeo (first time upgrading OS on a blackberry), that's what I would recommend doing.
Just keep in mind that doing the .148 or .151 OS upgrade via Desktop Manager yourself poses no real danger with either version. By that I mean, you run the same risks whether you do .148 or .151, so keep that in mind. The only real difference is that one is supported by Verizon and the other is not. So, if you're in the habbit of calling their support every time you think there's a problem, then you're going to want to stay with .75 or .148. If you do go with .148, you will lose color depth and cleanliness of the interface as apposed to .75, but other things will be slighly better as well as other bugs will be introduced (such as HTML email menu bugs and such)... bugs that appear fixed in the .151 release. So, make an educated guess and go from there.