I think you could do a bit of both, since it's usually a good thing to communicate with many people and spread the understanding of how the system is built.
Choose reviewer based on the type of code under review (if it integrates heavily with the back-end, use a back-end dev... if it's mostly front-end, see about using the other guy.)
See if some back-end guys are more willing to learn/invest in the apps front-end than others. That's usually the case, and it's good to have someone in the office know abit about your front-end, to mitigate the bus-factor and help with workload peaks.
Code reviews can be done "offline" (i.e. via e-mail or web-tools) to mitigate the time difference. If you and the other UI guy are communicating well already, it'd probably work. If not, you both should discuss if you're comfortable with the idea and willing to spend the effort required to get there. (i.e. to know each other well enough so that you don't need to communicate in real-time.)
This depends on the other people being willing to do spend some time with you. You probably have to drive this a bit yourself, but see if you get permission, support and blessing from your (and their) boss, as it will make things much easier for you and everyone else (since they know it's ok to move some hours from their own work to code reviews with you).
Hope it helps.