I have been training since August on a mtn bike, just prchased a Giant road bike today, is it wise to attempt the June ride on it or stick with mtn bike?
You should ride with whatever you are most comfortable with.
You still have 3 weeks to get more familiar with the road bike, I'd stick with that over a MTB myself.
My first ride was on a Giant hybrid that I borrowed from a friend last minute because I didn't want to use my MTB, and I'm glad I used that instead, especially going up that hill in Hamilton at the end of day 1 :)
As long as the new bike is set up properly for you and you put in some decent rides before hand you shouldn't have a problem.
I had my hybrid stolen 6 weeks before the my first ride. I had about 4 weeks to prepare on the replacement road bike. With in 3 weeks I did a 100km ride with no problem.
Are you doing the 100mi or 100km route on the first day? I've heard of many riders doing the 100km with no or minimal training on all sorts of equipment. I don't recommend it, but its been done. The 100mi on a mountain bike has been described as torture even by people who have done preparation.
Thanks for the replies everyone. I am doing the Toronto to Niagara classic run, first time, pretty excited. taking the bike for a "fit" this week. I was really only worried about the seat/saddle, i had no problems whatsoever with the mtb seat.
Or just spend all the time you can between now and the ride on the road bike. And get a high quality pair of cycling shorts, with a nicely padded chamois.