[...] which one is better?
If they both have the same ERP and all, you have to look at what environment it will be used in. Both are ISM bands, so if you know someone with a good scanner (perhaps an exprimentin ham operator), scan both bands to see which is the most "silent" in your area.
The higher the frequency, the less "penetration" of walls and other obstacles.
The lower the frequency, the longer the antenna needs to be.
The EMC generated by your robot may have a specter that will make one perform better than the other.
If the 315MHz unit is bought too cheap, you may get some Chinese crap that don't hold specs. (I have a few 433MHz modules that has got a range of less than 10m - they were spec'd for 100..150m.
Whether it's important to you or not, there may be vast differences between the max. data rate.
In the end, only you can make the decision, as you're the only one who knows all the details that goes into making it, but if you go with proven technology (read user reviews, not at the sellers site though) from a trustworthy business, you should be good.