Damn, tough choice! All 3 are great. Gaiden is my favorite Gradius, but I really like what Konami did with the Rebirth version. I really wish they re-released the Rebirth collection 'cause all 3 (castlevania/contra/gradius) are excellent.
I also just bought a Twin Bee PCB too! Hard decision, but I think Gaiden needs to be played more. Plus we can run it on the MiSTer through the PSX core.