If you are going to be staying that much then the most important thing is that you stay at the most comfortable property with the best staff/service (and location). It may be worth trying a few properties out first to see which one suits you the most.
I would also recommend contacting the hotel managers and explaining your situation to them. 3 weeks every month is a long time and a lot of money for them so see if they can arrange any special perks. One loyalty scheme may appear to give more perks but a competing property may offer more to you directly.