I guess it'll come down to how you want to use it.
If you intend any highway travel the 3a is probably only good for 45 to 50 mph, maybe less if you go for long distances. Even with an overdrive the 5.38:1 rear end keeps the road speed down.
But you can always flat-tow it for reasonable distances.
Nothing is more fun off road on narrow tracks than the 3a, especially with the windshield down.
3a parts are readily available, but mostly mail order. Things like oil filters, spark plugs, points can be found (or ordered) through local parts stores.
Lots of repair and maintenance advice available here and through available manuals.
A relatively modern Commando will be more comfortable. Probably has a pretty good heater and maybe even air conditioning....and it will do reasonable highway speeds....maybe not the 80 mph they do on the freeway here but probably 65 easy enough.
Of course you could easily flat-tow the CJ-3a with the Commando...that would be the best solution!!
Randy