Depends on your power requirements really. If you can vape comfortably with 15 watts or less, and be satisfied, I would recommend spending your money on redundant 2.5's, because in my estimation the fixed pin 510 connectors have a better chance of outliving the P3 connectors which incorporate an O-ring to support the center pin. That being said, the P3 and Procyon can utilize flat top cells, so they will be more versatile in that respect. ( there are hacks to get around that if it's ever an issue
) With several 2.5 minis, a few extension rings and maybe even an extension cap, you can utilize all three 1800 series batteries. Decisions, decisions...for me the best bang for the buck is the closeout 2.5's, but I vape at 7-8 watts all the time...