Generally speaking, I would split the difference a little bit; you might want some gain on some bands, but not on others. I would not want too much gain on 800, perhaps a bit more on VHF hi and maybe - just maybe - VHF lo band. I'm not too familiar with that area you mention, but at least part of the Expressway is on the very edges of the hxxx area. Remember - older scanner, and no matter how bulletproof you think the front end is, chances are that when you're in an urban area, a gain antenna is going to give you grief. That grief could show up as pagers blaring, radio going dead, choppy reception on certain bands...there are too many symptoms to be sure.
Let's see, Carroll and Baltimore counties both have 800 systems, but York? The last I had heard, they still mostly used VHF hi (I haven't lived in Pa. in almost 20 years now, so don't take that as gospel), tho their fire stuff is still low band. I would think you'd want some lo band coverage, if for nothing else, to get the 47.32 and 47.40 SHA stuff (particularly important when the white stuff starts coming in ernest). I think you get the idea...of course, when Pa. shifts to Open Sky (yeah, right) all that goes out the window...
I just can't bring it out of the old EPROM right now, but there was something quite a while ago about a slim antenna that everyone was saying worked quite well (MFJ?? comes to mind...) Personally I have my doubts about something that small working at all on VHF lo. I'd also take a look at Northwest Radio's survey in the Antennas wiki, as well as some of the other links found there. Personally I like the Austin simply because it's so damn tough; it's been through several ice and snow storms with little more than a few scratches for the trouble. You can even cut the top whip for better reception on VHF lo band. I did, and it did make a somewhat noticeable difference (I cut it for 39 mhz - guess why? Awww, you peeked....). It's expensive but worth it.
If you can mount it on top of the roof with a good thru the roof mount, that's best. A trunk lip mount, like I use, will work just fine also, but not quite as good. If you use a trunk mount, make sure that the mounting screws bite right into the metal - the Austin needs a good ground. You can easily check this with a simple VOM - if you have a good path to ground between the mount and the car, you'll get a (nearly) full deflection on the meter. (I say nearly because there's almost always some resistance in the circuit. If you were to transmit through the antenna, I'd say use a couple of braid straps, but they're really kinda overkill in a receive application).
Mag mounts? Yechh. I avoid them like the plague. Scratches up the car, and as the magnet weakens over time, so too does the ground connection (which is capcitively coupled). Good for a quick fix, but not for a permanent solution.
In the meanwhile, let me see if I can remember that mini antenna everyone was harping about. I'll do a little digging tomorrow. 73s for now...Mike