Roof mounted antenna's can be a mixed blessing, especially with a radio like the Pro-95. If you live in a large population area, you are likely not to receive local signals any better than with the standard rubber duck but you will receive signals from other agencies on the same frequencies as your local agencies that may even be strong enough to interfere with the agencies you want to listen to. Without CTCSS, there's no way to block these signals. You'll also get more interference from things like pagers and intermodulation from other frequencies.
If you live in a rural area, a roof mounted antenna can be a big help getting more and stronger signals without all the problems you'd have in a metro area.
As to the antenna, the answer really depends on what bands are the most important to you. If you need to listen to everything from 30 mHz to 900 mHz, a discone antenna is about the only reliable solution. If you want to concentrate on UHF or VHF, you can get relatively cheap gain antennas that will perform better than a discone but you won't get frequencies above 500 mHz as well as with a discone. Thus, if you listen to a mix of trunked and conventional comms, you should get a discone. If most of your listening is in one or two frequency bands, you're better off getting an antenna that is made for the those bands, accepting decreased performance on the other bands.
There's a simple rule with antenna height - higher is better. If you are in a rural area and trying to listen to stations two or three counties away, a 40 foot mast would be ideal. On the other hand, if you're in a metro area, an antenna on a 5 foot mast attached to your chimney may give you just enough increase in range without all the overloading and intermod you'd get if it was mounted on a 40 foot mast.
As you can see, there are no cut and dried answers for the "right" antenna. Everything you do will be a tradeoff so you'll have decide which tradeoff's you're willing to accept.
