You are just starting to ride regularly, is that correct?
That's correct today was my first commute and only been a leisurely rider every now and again before that.
I remember when I started bike commuting again, I was rather unfit, and was huffing, sweating and red faced, in the small chainring (of a triple) at 6 mph up a particular incline.
Yep, that sums me up today.
Depending on how full the money tree is, I wouldn't put new tires on the MTB, I'd use that money on whatever the road bike needs, or on lights (safety) and rain/cold gear (comfort). Because once you get the road bike running, you won't ride the MTB for the commute anyway so the slick MTB tires will be wasted.
Oh by the way my rule of thumb is
1 - MTB with slick tires is 2 mph faster than MTB with knobby tires
2 - Road bike is 2 mph faster than MTB with slick tires
at a constant moderate effort level (like a fast cruise) on flat roads.
#1 is because of the tires,
#2 is mostly because of the riding position.
Yes I am considering my options carefully and very much on the look out for a road bike (other than the project road bike I have).
I do have a question around the safety of the road bike and tyres in the winter months?