I wouldn't personally get either of the devices you stated -- I'd probably get a 4350, because I want the integrated keyboard and larger battery.
That unit aside, I'm competely torn between the 4150 and 5550. I like the expandability of the 5550 (I already have sleeves) and 128MB, but I've held a 1910, and that form factor (which it shares with the 4150) is simply amazing. So I won't be of help, I suspect.
--janak