Pardon the wordiness of what follows, but for what it's worth, I use a magnet mount Wilson 5000 antenna (here:
Amazon.com: Wilson 880-200152B 5000 Series Mobile CB Antenna with 62-in Whip: Electronics) (although if you're not adding any power, the Wilson 1000 works just as well and is cheaper - it's just got a smaller coil inside) coupled with, for now, an older Radioshack (Realistic) hand held/car adapted CB Radio unit that is no longer for sale but equivalent to this one from midland here:
Midland Micro 40 Channel Hand Held CB Radio - 75... : Target
The nice thing about my setup is that it is 100% portable, can be removed completely without issue or stashed in the truck out of sight (with clever stowage of the whip), and still performs great. The Wilson antennas are widely considered the single best magnet-mount mobile antennas and come with trim-able, removable, stainless steel whip antennas that allow you to size them for the perfect SWR for the 4Runner, mine is seriously damn near 1.0 at Ch 1 and Ch 40, but not quite there. I could trim a bit more and probably get dead on - but this depends entirely on exactly where you mount the antenna on the roof. I put mine top dead center and can send and receive loud and clear for around a 10 mile radius in most cases, occasionally I get some pretty wild skip from other states, but that's another story. There are some better mobile antennas out there, however, but they are not mag mount.
If you DO go for a magnet mount antenna, be sure you're using at least ideally a 62'' solid steel whip antenna mounted ideally as high as you can, and place a custom cut buffer of panty-hose or something between the magnet and your truck to avoid scratches from bumps in the road (this has worked wonders for my application) - also, before doing anything, clean and wax the area you're going to mount the antenna on to avoid any damage to the paint. The magnets on the Wilsons will hold solid on the freeway and even if you happen to hit tree branches and stuff with the whip (it will just bend and remind you it's where with a noise). They've held solid while wheeling, too. The cable tucks neatly into the door of your choice, or you can do some drilling if you feel so inclined. The Wilson 1000 and 5000 unscrew from the magnet mount and come with a weather cap to put on there when it's taken off so you can just take down the antenna when going into the garage, etc and stick it in the truck or whatever. You can also start with a much cheaper, and smaller wilson/k40 magnet mount antenna, but you will NOT get the same performance out of them, although on a budget the little 'Wil isn't a bad buy to test out your radio at first. I would encourage you to avoid the K30/K40's, as in my experience they didn't hold up well at all and were vastly outperformed by the Wilson products, but your mileage may vary.
The firestiks are great, and I have many buddies that swear by them, but they do not work that well, in my humble experience, with any sort of practical magnet mount setup. You're gonna want to mount those on a bumper, hood clamp, or perhaps a custom roof rack setup if you feel so inclined. They hold up quite well over time, though and take a fair bit of abuse as long as the mount is done right. They're not a bad antenna at all. Also, some people think the Stainless Steel Whip Antennas make their vehicles look like a giant RC car. I personally don't think so/care, but if that's a nitpick of yours, the firestik type is the way to go.
Hope this has shed a little light on the issue for you. Let me know if I can clarify anything, etc. I love my CB setup, always listen to the truckers yammering on the freeway, etc. Occasionally hear some pretty hilarious things and have made some good contacts in the summers when all the ORVs come out to play. If you like the CB, you might one day consider getting your HAM license (It's nearly free, only requires a little bit of study) and getting a 2m/70cm setup installed at some point to make contacts far further than the ~10-20 mi line of sight that CB radio allows.
At any rate, hope you enjoy. Again, let me know if I can help.
