Bottom line - if u need a folder to make the commute happen, get one

I hear there are excellent light weight folders available now, so I would recommend splurging on a good one. Remember the axiom for bikes - Light, strong, cheap - most bikes will be any one or two of those but good luck finding one that is all three.

In the end if you spend just enough for a decent bike you will be rewarded with a better ride, and more pleasant feedback. Last time I checked good light folders were around the $1000 mark - but I could be wrong....

fwiw, road bikes on average are usually in the range of 20 lbs - pro bikes like the ones ridden in the Tour de France are closer to 15 lbs, the average crappy old 10 spds you see rusting away everywhere are probably 30+. A decent Aluminum frame road bike (about $750 new) should weigh in around 25 pounds max.

MTbs are rarely below 25 lbs, unless you are looking at very expensive pro bikes - and even then, suspension adds a lot of weight.

But it is the rider position and tires that also affect the ride. I commute in a city setting so a mtb is best for many of us here. I only go 6 miles each way, mostly stop and go for lights. I raise the seat and lower the handlebars to get a more road bike-like position, and I changed the gearing for a faster ride. Also, my mtb is a decent model (about 25 lbs) with no shocks and very narrow tires (1"). But if I had a longer commute along highways, I'd use an old road bike with drop bars.

It sounds like you should get a light folder for commuting and make sure you get it at a good local shop that will fit you well and set the bike up properly. Stick with it and you can always buy a new or used road or mtb later as needed.