I recently converted to sms popup and I like it better than handcent. I primarily switched because a) handcent's popup was a bit laggy and b) handcent would not pull my contacts' pictures. Sms popup is different in that it ONLY creates a popup notification, and redirects you to the original sms app. Handcent, on the other hand, handles every aspect of mms/sms on its own.
If you like the speed and using the original sms app, go with sms popup. If you really need voice-to-text and other advanced options, then go with handcent.