Most of us here don't have enough knowledge or first hand experience with the Avenger to really give you an answer.
Engine vs. Engine, the Phantom is prone to running hot, but if not pushed hard everywhere will be just fine. I've been running around with mine for over a year and it's still running. No needle bearing failures or meltdowns to speak of. Mine is also a needle bearing V2, aka V2.1. The V3 has a good track record of reliability, especially compared to the previous generations.
THe Avenger looks to be a good engine and will likely run cooler due to more fins and fin area. Since no other useful information has been released on how much power it makes, or head temp data based on how it is ridden, I can't speak to how it runs.
I think you would like either engine in the long run.
Without more information and knowledge on the Avenger I can't give a fair comparison. Buying the Phantom through B-E will also give you a level of warranty coverage not offered from most other vendors. They do honor their warranty as well - speaking from personal experience.