Question about which bike is better for commuting

Hello, I am new to these forums and reading all these posts has convinced me that I want to start riding my bike to work! Especially after we hit a record high temp yesterday of 82. I know the snow isn't over yet but it should be soon.

I have an easy 5-mile commute that is mostly flat to downhill, leaving the sweaty part till after work luckily. I can also follow my bus route, giving me easy transport to work if I get a flat. Traffic is pretty light except for a few city blocks at the end. I'm wondering which bike I should use. Just got a used road bike and though it was inexpensive, it looks brand new - I already want to baby it. I have a 15-yr-old Specialized Hardrock that rides just fine, would that make more sense? I could add a comfier seat, lights, a rack - all stuff I don't want to put on the road bike - and not worry about padded shorts or clipless shoes/pedals since the ride is so quick.

What works best for you ladies? Thanks! :)

Will you park the bike inside or out? If outside, then I suggest the Hardrock fitted with slick tires and racks and lights.

I would use the mountain bike with slicks and a rack.

Yep I'd be parking it outside. This is sort of what spawned my question - I see a lot more beat-up bikes parked outside my building. I'd feel less stressed about my old bike in that case. Slick tires are a great idea and I hadn't even considered it. Thanks so much for the input!

I ride my race bike into work all the time, it's just more fun.....BUT.....

I'd never ever ever park it outside (it sits in my office)
I don't have a need to carry much (backpack with the computer and change of clothes)

Some things to consider:

What are my storage options (outside, inside, bike lockers, simple racks, secured, unsecured)?
What do I need as far as carrying capacity (messenger bag/backpack, rack and basket, rack and panniers)?
What kind of rider am I going to be (happy to be riding to work no matter what, annoyed that I'm going 2mph slower than I could be on the other bike, etc.)?
What kind of weather am I going to ride in? (Rain=fenders necessary, Snow/slush=clearance for studded tires necessary, braking considerations for both, Night/dusk/dawn=good light set (probably semi-permanently attached to bike) necessary).
Will I want to get on the bus with the bike on a semi regular basis (what bike am I comfortable doing that with)?

At first glance it sounds like putting slicks and maybe a rack system and fenders on the mountain bike is the better option from my perspective, but as you can see, what I advise and what I actually do are two different things (I simply like to ride my race bike that much more, regardless of how much less convenient it is. Perhaps this is a sign I need a better commuter ;) )

I *am* a little torn just because my road bike is new to me, gorgeous and so fun to ride. I'm thinking why ignore it and go back to my oldie but goodie? But you're right, I don't want to stick it on a bus rack if I don't have to, much less lock it up outside my building. I don't carry a lot to work either so maybe a messenger bag would be enough. I probably won't ride in rain/snow at least for now. I won't be frustrated going slower. So...my current mountain bike set-up is probably decent if I swap out the tires and add lights.

This is really helpful, thanks everyone!