I've always heard UHF is better for urban and congested areas as it penetrates buildings better but vhf is better for open terrain and hilly areas as its longer wavelength always it to travel further in range. so my question is if your camping or hiking, what would be ideal in an environment that has alot of trees and is also hilly? UHF or VHF?
The real answer is "It depends", and that is based on many factors that vary from one area to another. The real answer could be what repeater has the best coverage where you and who you're wanting to talk with are (with linked repeaters, the answer may even be both with you on one band and who you're talking to on the other). With simplex, it could also be different depending on what conditions each of you are under at the time.
While I understand your desire for a single, works-everytime answer to your question, this may not match the reality of the many factors that will be at play. The ideal answer is probably for you both to have dual-band radios with pre-programmed frequencies on both bands and have a pre-arraigned schedule to check them. If the VHF frequency won't make it, perhaps the UHF one will. Hopefully your radios will allow them to scan both bands so whichever the call goes out on you'll hear it. If not, you may have to manually switch bands often so you can catch any calls made on the other band.
The important thing is the "schedule". This doesn't have to be anything really fancy, but should be often enough that if there's trouble, help can be gotten without too long of a wait. Perhaps something like "contact me every xx mins, first on channel 1 and if no answer, on channel 2 and keep trying, switching back and forth until contact is made" and channel 1 would be on one band and channel 2 on the other. The timeframe would be adjusted based on your activity. If both are in camp where safety is generally assured, this may be several hours, but if doing more risky activities (hiking in unknown areas for example) shorter timeframes, say every 15 mins or so, may be more appropriate.
You both should also know to try to improve your locations, if possible, when contact can't be made. This may be by moving to higher ground, away from dense foliage, etc. If you can have repeaters programmed in as well as simplex frequencies this may be helpful. Just increase the channels checked so all are checked when contact is desired. The repeater(s) may allow you to monitor multiple frequencies as well by asking local hams to monitor for you and passing messages that contact is being attempted. This could allow one party to remain on the repeater frequency until the other party returns after trying the other programmed channels so you don't play "frequency tag".