Weekend only activity, as has been noted, makes it more important to get your hours during the training period. However, it's not a killer.
Should you join Army MARS, for example, you will have plenty of daily nets (typically, at least, two).
In fact, just in Navy-Marine Corp MARS, on Saturday alone, I could do one of three nets during the day. Air Force has a nice two-hour net on Saturday morning.
Since I am in NAVY-Marine Corp, I can also say that there are plenty of evening nets. In other words, you can get on the different State nets to get your participation time. (Just one difference, if I understand right, between the services. NAVY carves up the big State of Texas into three parts with a "State" net for all three.)
Join the MARS program that suits your interest. If it's ARMY, do it because you are interested in ARMY. There's plenty of activity going around.
I am seeing more Army and Air Force stations checking into NAVY nets (and vice versa). It adds value to the interoperability factor when military units come to help during a disaster.
I hope you have found this helpful.
Lloyd Colston
Pryor, OK USA