I haven't noticed any serious time rendering differences between Twixtor and Premiere Time Warp actually. I was using Twixtor until CS3 arrived and from that moment I stayed with Time Warp. An advantage of Twixtor is the wealth of options to experiment, but if you use it as it is, it creates a heavy watering effect at certain surfaces, while with Time Warp the effect is minimal (or non-existent). For example, watching Baptism in Thesprotiko on Vimeo
you can see the Time Warp use, with the -almost- default settings.