Cyclocross - convert a road bike or a MTB?

I'm trying to learn & understand more what my options are for achieving the best bike for commuting cross country to work in the spring & summer as many days as possible. The distance is about 15 miles & is all pavement but bot pristine pavement. I recently became aware of this bike class & I'm thinking my commuter should have cyclocross attributes but I'm not sure which bike to work on. Both are old beaters so I can pretty much do whatever ever I want. So what's your opinion & why? Roady or MTB?

That would be about a 60 minute ride time to commute.
Ride both.
Which would be the fastest?

Pavement = road.

Get a nice new pair of slick/semi-slick tires in the 25mm to 32mm range. I like the Panaracer Pasela for that sort of duty, but there are plenty of other choices out there.

10 wheels - yep I've done that. And you're timing is dead on BTW :thumb: The MTB has the record time of 50 minutes. The old roady is consistently over an hour. I need to do another time trial though because there are some variable factors I haven't ruled out yet. But I'm leaning toward the MTB.

But you're saying go with the fastest ride. OK - that's useful info and jives with my thinking too because I always want the shortest time possible. It is work after all and the best option is always the most efficient option.

flargle (http://www.bikeforums.net/member.php?u=94092) - I already have slicks on the MTB. But I like the looks of that tire!

I ride a road race bike on my 7.5 mile commute.
You want to have fun and also get there. Yuk ! Work !
I am retired.
Sounds like you have a strong motor.

I am "working-retired" Ha! When I'm strong, I ride. When I'm not, I drive. I only did 1 complete week on the bike last year. 170 miles ...! Most weeks were just 2 or 3 days. Looking to improve that next spring.

Get a nice new pair of slick/semi-slick tires in the 25mm to 32mm range. I like the Panaracer Pasela for that sort of duty, but there are plenty of other choices out there.
+1
Excellent choice for a commuter tire.
I had Panaracer Pasela TourGuards (700x32) on my cyclocross/commuter bike for 3 years. Recently swapped for 700x28 TourGuards.

Theses are nice , especially if you get caught in the dark.

http://www.nashbar.com/profile.cfm?category=6000121&subcategory=60001254&brand=&sku=25340&storetype=&estoreid=&pagename=Shop%20by%20Subcat%3A%20700x18%20to%20700x28