It seems the consensus is the MIIIFS for stapling duties. I've seen that people are shimming the 1/2" foot for use with the 5/8 flooring, but what about the 3/8" engineered flooring?

Also, for some reason I'm scared to death of buying a MIIIFS and then splitting the tongue consistently...any thoughts on this?

I'm planning on being in the house for 5-7 years, so 3/8 seems like the way to fly, but because my help (a relative) is only in town for 2 days, I need it all to go off without a hitch.

Thanks for your thoughts.

The M111 FS uses a rather large staple that is generally too much for a 3/8" thick floor. I've seen it done but I've also seen a lot of damage caused by using that nailer on a 3/8". Recommend the Floor Runner or similar instead.

Never seen a thin 3/8 that was recommended for fastening. All the ones I have seen, were gluedown only.

apparently they do exist:
http://www.hardwoodinstaller.com/hardwoodinstaller/greenwood-engineered-horizontal-carbonized-48.htm
this is the one I'm looking at.

3/8 and even thinner can be stapled down ( Bruce Natural Reflections is 5/16" ). In fact, many of Bruce's 3/8 floors recommended stapling with the Floor Runner instead of gluing if possible. I don't recommend stapling a wide plank, THIN floor however. Gluing does a better job on thin floors but if done correctly, stapling is fine.