I think Futian is probably the place to go - the crossing from Lok Ma Chau is much quicker than Lo Wu but not much difference in time from Sheung Shui. But you would save alot of transport costs if you just live in Sheung Shui (the MTR to the border is hyper expensive - but not HKD30 as suggested - I think it's closer to HKD22). I'd take a look at small studios in Sheung Shui first if I were you before making a decision. If you are young you're not going to care much about having a big place! Better to have more free time.