Depends. I used both GMRS and CB in the interlake out near Lac du Bonnet.
My friend and I have CBs in both of our vehicles, and a base set up at the cabin.
We have yet to find a spot out of range with the CB, its handy when one of us drives into town and we need to get ahold of them. (not all of us have cell service out there.)
We get better performance out of the CBs than GMRS operating at legal power. One evening I was chatting with my friend from my truck just after turing onto Hwy 317 off of Hwy59, he was operating the base at the cabin on CH 38 LSB. Keep in mind though we have a fairly large 5/8 wave antenna fed with low loss RG8U cable at the lake on the base. We switch to AM and take the handheld CBs on the boat when we go fishing. Most of the time I dont have to switch the handheld up to full 4w power.
GMRS is alot more portable than CB though, CBs are fairly large, and the size of the antennas may not be practical. they do tend to chew the batteries quicker if you run at full power. My GOOD handheld CB is an old X-tal controlled unit, 6 channels, with a large telescoping antenna nearly 4ft when fully extended. It weighs a bit, will not fit in your pocket, (its about 3"w x 10"h x 2"d) and sucks off of 10 (yes ten) AA batteries, it does however put out the full 4w legal max of power) My medeoker one is a smaller (but not by much) radio, PLL controled full 40CH, and a 12" rubber duck antenna. Sucks on 9 AA batteries. It claims full legal power but the crappy rubber duck antenna just sucks. The full telescoping works way better but is very unweildy.
If you use GMRS stay off of the FRS channels as they operate at significantly less power than GMRS.
Best thing to do is try a pair of GMRS radios, dont bother looking at the range printed on the package. try to find out the output power of the radio, buy one with as close to full legal (4w) as possible.
The antennas on those radios suck, they are poorly matched and low gain (if any gain at all).
I keep my GMRS for use around the lot, or if there is alot of CB skip activity. Otherwise CB seems to work well for me out there. There is also usually noone on the air up there.