i like 3.52 M33 for psp phat better and 3.71 M33 for Slim.  

3.52 M33 doesn't have any problems in flashing themes and is safer than 3.71 M33.  there are cases of total brick on 3.71 unrecoverable by the pandora without a backup of your nand.  

i went through this unrecoverable brick myself but i had a backup of my nand so i was safe.

3.71 M33 is also required for newer games, it also has a nice theme feature even if they didn't go far enough to make it as good as flashed themes.

currently I'm switching between 3.52 M33 and 3.71 M33 from time to time.  ((thanks to my nand backup :D))