321p is about 430's to 450's. Check e-bay cause sometimes they have great deals thier. Man, i had one NIB for sale and i was only selling it for under $350.00..
Check this shops too,
www.microrcshop.com or Ace
Regarding the difference, 321P is ABC while 421P is AAC. Both are 8 port engines. Get the 321P since it last longer.