Boils down to the right tool for the job.

I have done what you are doing and ended up on a road bike like most commuters. The exception is if you do a lot of urban commuting on poor road surfaces. The Mountain makes a good winter bike I will give them that but summer if you have a moderately long commute (3+ miles of road) a road bike will leave you in the dust.


Let me ask you this:
Is your commute going to have you off pavement regularly or on rough roads? If so then go for it. It is a better option.

My comute is all street and as such I rapidly found that while I LOVE my M400 it had limitations. The 42/32/22 crank sets were great for the hills but I was always spinning out like you are finding.
Spinning 1.95 tires takes a LOT of extra effort even with a road biased tires. The difference between a 550gram tire and a 350 gram tire is a lot more then you would think.

Then we get into hand positions. I went from straight bars to riser bars to trekking bars trying to get my wrists to keep from hurting even after 5-6 miles. None of them could over come the gemotery of the bike.

A mountain bike starts by raising the bottom bracket for better ground clearance. That is a major Geometry change that is always a negative to street riding. I am a little over 6ft and my M400 is a XL (22inch) frame. To get the correct street riding postion to maximize my efficiency I ended up having a HUGE amount of seat post out. My Mountains seat height is close to three inches higher then my road bikes. because of the BB and seat tub angles.

Now I am at the extreme and if you are say around 5' 8-10" you may run into less problems then I did. Mountain bikes size smaller to give you stand over clearance on uneven terrain.
The older the Stump jumper the closer the geometry is to a Touring bike so this may become a mute point if you are running a early 90's model. So by ridding as tall as you can with a couple fingers of clearence between the top tube and your pubic bone you may not run into the same problems.

Here are the advantages of a road bike over a mountain when both used on the road.

Lower BB= lower Center of gravity.
Lower BB and drop bar= lower wind drag.
Drop bars= more hand positions to elevate wrist and hand discomfort.
Drop bars= Better aerodynamics.
Lighter thinner tires= less effort to sustain a given speed.

Here is what my M400 looked like after it got the Trekking bars. Notice how high the seat is?