Now that the main points have been covered (take the class, do NOT plan to use it on your dives, etc) I'd like to bring up two conflicting points.
1) I'd rather have a small tank with me on every dive than a larger tank that I leave behind on most dives. If it's small and convenient, I'm more likely to carry it. If it's bigger and less convenient, I'll be less likely to carry it.
2) Have you figured out how long it'll actually keep you alive? I did some quick math. The big unit (3cuft) has 2.5ft3 of "usable" gas (giving 500psi reserve, about 35bar). Assuming a SAC rate of 0.65 (pretty common amongst rec divers), and a dive to 100ft, and ascending immediately upon switching, you get 90 seconds of air. This is only if you don't have to purge the mouthpiece with compressed air, take a big gulp of air to recouperate, and then have an elevated breathing rate upon ascent. Also, this does not include spending time at depth doing anything. Including all of that, you conservatively get no more than 45 seconds of ascent. An ascent from 100ft in 45 seconds is extremely unhealthy, and is 2.88x the "recommended" ascent rate. If you take into account common "elevated" breathing rates, your numbers plummet to 25 seconds of ascent.
So, to answer my first point....your buddy should either be well ready to donate to you, and planning for "turn pressures" should be made, or your pony had better be bigger than a Spare Air. With a little tweaking and persistance, you should be able to sling an AL40 very comfortably on every dive. I know that's not necessarily relevant to your question....but it's something to think about.