Well one thing is for sure, they are both going to be a lot more power than what you have now. I think the T5's will choke you up some on the bigger motor. I personally do not know anyone that dunes a 421, but I have seen people that do. So maybe someone can chime in. It is a proven reliable drag motor.
The Serval is no joke when it comes to HP, but it is not a drag motor. Like BBC said, not much feed back yet. But the Dyno sheets look bad ass. As far as drags go, looks like it will be a good match up with some stock 4mil alky bikes, or some stock stroke PV cheetahs. However, the Serval is more about a usable power delivery not just WFO. I know when in the sand, I dune 100% of the time, so the Serval would be for me. But Since it is also fast as hell, I would start to get into drags....... and that would probably lead to building a bigger motor. lol fuckin addiction.