Did you choose those two species cause of the low price? Just wondering as both of them have the cheapest starting price. But be warned you'll spend hundreds or thousands after your initial purchase. i.e. food , cage, and possably vet bills.
If you've already looked into that, then from what you say it comes down to looks? This will be a personal preference. But keep in mind sav's are all basicly the same in color and only vary slightly. Niles color and pattern has alot more variations.
I have some extremely yellow niles and some real nice blue niles.
The other thing to come into play would be size and attitude. Savs are smaller and less likely to be stubborn and offensive. They may become defensive tail whip hiss. But the nile also has a good offense of charging and biting.
I suggest you study up on both and think about what fits YOUR needs.