Welcome to MBc, Mauriel. Mucho gusto. Since PR is mostly mountainous, I would pick the bike with the best front suspention, and the best rear suspension, (or a suspension seat post)...money well spent. I'd buy a 4-Stroke engine and positive gearbox...such as that custom milled by Dave Staton at www.staton-inc.com. He will be more than happy to help you out. Be sure to specify heavy chain, but mention that to Dave Staton and he'll know what you need. Good luck to you.
Welcome to the forum. If one of your mountain bikes is steel framed, I would choose it to motorize. If they are all aluminum, choose the one with the best brakes (if there is room in the frame for a motor). Just my opinion.