It would depend on whether you can record better than the other. Some people stick to 5 mora because they can't handle recording in 7 mora. If you run out of breath easily from recording 7 mora, stick to 5 mora. If you can easily handle 7 mora, I'd recommend using 7 mora since it has less samples to record than 5 mora leading to a smaller voicebank size. Or even better, if you can handle it, try 8 or 9 mora.