My personal feeling is the B20a, because you can boost it without killing your rings and it is reliable under boost when built properly.
The B21 has proven to be a good motor as well but it is way more temperamental, and needs more attention to keep it reliable. Even after a rebuild the FRM sleeves just eat thru the new rings that are replaced.
There have been other guys who relseeve the B21, but at that cost what is the point, you might as well have just picked up a B20A5 cheap at a junkyard or someone who is selling theirs.
The B20A5 came with factory sleeved iron sleeves, and are proven to handle 400+ HP. (just the block, the head will need to be strengthened)
Does that answer your question?
Oh and one more thing, dont think about going N/A build on a B20/B21 motor, the most you will get for spending $3000 will be around 150whp. Boost is the only way to pull any real power out of a B20A5 or B20A(JDM) and the B21A1 as well if you choose to boost that motor.
3GP Aftermarket Specialist.
Sean Tydingco