There's a tradeoff to be had - while lowband will carry farther if all other factors were the same (antenna gain and height, power output, etc) it is much easier to (for example) get a very high-gain mobile antenna for 2M than for 6M - because the antennas are so much bigger at the lower frequencies.
So, as long as you don't have very tall hills and deep valleys, 2M could outperform 6M distance-wise with some good high-gain antennas on each end. (Certainly my experience here.)
On the other hand, 6M will get into the valleys better, so you'd have better fill. You just need to have a decently-large antenna (1/4 wave on 6M is over 4 1/2 feet tall!) to really work well.
There is also a lot more noise on the lower bands, 6M tends to drive me crazy when driving through town as some areas are so noisy there I have to crank the squelch all the way up. Of course, then I can't hear anyone transmitting there either. The higher the band, the quieter they get - in general. PL can help there, if you are using a repeater or only listening for others who will know to use the tone. I'm generally listening to the call channel or SSB.
I like UHF better for shorter-range stuff that needs to penetrate buildings and such. The antennas are also smaller, or just much higher gain for the same size. But it doesn't do well if you need to get long-distance, unless your base station can have a VERY high antenna. (A couple of UHF repeaters here are up around 1100 feet on a TV tower, and they get some wonderful range - still not as good as the VHF ones along side them.) Get behind a hill at any distance away, and the signal will be gone!