If funds allow, I'd do both.
1. Get a proper dual band HT antenna, instead of the stock rubber duck antenna. I've had good luck with the Diamond SRH77CA. I have 2, one for my Yaesu VX-170 2m HT, and one for my Yaesu VX-7RB. I haven't tried it yet, but supposedly also adding a counterpoise can help. Counterpoise should be 19" for 2m or 7" for 440. Cut a piece of wire an inch longer than recommended, strip an inch of insulation off the wire, and then wind the bare wire round the antenna connector, or fit a ring terminal over the inch of bare wire, and slip the ring terminal over the antenna connector. In both cases, screw the antenna down onto the bare wire or ring terminal making sure the counterpoise has a good connection to the antenna ground.
2. Get a Larsen NMO 2/70B (the black version with the open air coil), as suggested by KDev, and put it on a mag mount with cable, and get the requisite SMA <--> SO-239 socket, so that the PL-259 plug on the end of the mag mount cable can screw into it. I am partial to the Larsen NMO 2/70B and have 3. The original one I bought many years ago, and has the odd chip on the base, and a few rusty spots on the whip, as it's been whacked numerous times on parking structures & branches & the like. Recently some annoying person bent the whip on purpose when it was mounted on my vehicle, but I bent the whip back, and it's mostly straight & still operating as originally designed. I decided to purchase a 2nd new one to replace my somewhat dilapidated original, despite the original still working, and then promptly mislaid it. So I got a 3rd, and then wouldn't you know it, I found the 2nd one again. So now I have 2 newish Larsen 2/70B antennas I can mount on my roof (so I can run 2 rigs at the same time), as well as a slightly cosmetically tarnished 3rd spare. It's also possible to get a Larsen 'SPRING-B' or 'SPRINGB' to convert the Larsen 2/70B to an antenna with a spring base. You connect the spring to the antenna base coil, and then cut the length of the spring off from the bottom of the whip, and then connect the whip to the top of the spring. I've bought a couple of springs to convert my Larsen 2/70B antenna to coil spring versions, but haven't actually done it yet & tested to see SWR is still good. I figured I'd try it on my cosmetically tarnished spare first, and then convert the other 2 as well if I like the results. One other bonus is that you can buy the Larsen 2/70B components individually, so if you lose or bend or break the whip or smash the base coil, you can just replace those components without having to replace the whole antenna. Anyway, the Larsen 2/70B gets good reviews, see here:
http://www.eham.net/reviews/detail/434?page=2, and operates as a center 1/2 wave on 2m, and 5/8 over 5/8 collinear on 440. A cheaper option if funds are short is an all-in-one cable/magnetic base/antenna, like the Diamond MR77. Despite there being an SMA version that can connect directly to an HT's SMA connector, I'd probably get the regular version & use an SMA <--> SO-239 adapter, as before, so the antenna can also be used with mobile rigs. I have one of each (regular PL-259 & SMA) versions of the Diamond MR77, and they seem to work acceptably. See here:
http://www.universal-radio.com/catalog/hamantm/3243.html. A full-size antenna will likely gain you several dB over a typical HT antenna, and likely more so if given a good ground plane as well. May not make much difference if you're close to the other person or repeater you're communicating with, but may make a big difference in locations where signal is marginal with a stock HT antenna.
Anyway, equipped with both a proper dual band HT antenna + counterpoise, as well as a full-size dual band antenna on a mag or fixed NMO mount, you could use the HT antenna when walking around or away from your car, and the larger full-size antenna when at your car.