I'm probably a little late to the conversation...
I'm a licensed HAM radio operator, and so have some experience with radios.
First off, Antenna, Antenna, Antenna... in radios, Antennas are the single most important peice of equipment... anything that can take an external antenna will be far superior to the little "rubber duck" type antennas found on hand held radios... and bigger IS better (generally speaking). Also getting the antenn high up, and away from obstructions (such as a rider) helps it radiate and extends the range. For example, I can take a radio with .1 watts with an external antenna and cover 2-5miles in an urban setting with a good external antenna... but take a 5w radio with a built in antenna and try and use it inside the car, you're lucky to get a couple miles out of it...
FRS might work, but not likely especially anywhere not line of sight (on a freeway straight line it might pull it off, in twisties, etc, forget it... you're only dealing with <1/2 a watt (effective radiated power out the antenna must be less than .5w, which means offten you aren't getting even that much)
GMRS is far better than FRS with a maximum allowed power of 5W for a portable/handheld radio, and 50W for a mobile... HOWEVER!!! GMRS IS a licensed service, and REQUIRES a license to operate on (this is Federal Law BTW!). yes GMRS radios are sold over the shelf, I Highly doubt any are actually 5w, and the main reason is GMRS has some shared channels with FRS... if you ever read the fine print in the book, you'll see it says you have to have a license for the GMRS channels.
CB is good, it's old technology, but works well... 5-10miles should be easily doable, CB your max is 5w at the radio, and you can put different antennas on... CB's effectively radiate more power out the antenna than an FRS/GMRS due to the mode they use (AM) With some CB's you can also run Side Band, and will get even greater range than in AM mode.
Option 4 (one most people don't know about) MURS (Multi-Use Radio Service) is a fairly new radio service, it was formerly a "business band" it runs in the 150mhz range (better range than the 400mhz range of the FRS/GMRS) allows a full 2W and you can use external antennas (if I remember correctly)... it is also a license FREE service like FRS/CB.
There are fancier options as well... you could both get your HAM radio licenses (fairly easy) and get 2M radios (144mhz) and run 5w on hand helds, with external antennas, or 50w on mobiles (there are motorcycle mobiles available now) you could also run a power amp on the portable and boost the power upto 50w (or higher!). in FM with a small antenna and a 5W radio you will easily reach beyond 10miles except in the worst of terrain. We constatnly talk 10-20miles to guys on handheld radios with a simple magnent mount antenna on the top of thier car. There are also a lot more frequencies available, you could go with lower frequencies for even more range, but the antennas get much larger...
My eventual long term for my bike is to get my HAM radio on it, even if it's in a tank bag that is only used occasionally. The versatility and range is just too good!
Take it for what it's worth... in good coverage the Cell will give the ultimate range, but you are reliant upon cell site coverage... you could depending on your phone do a bike mounted antenna or buy a cell phone repeater and antenna also to extend the range of the cell...