You might just stay at the Mercure this time since you've already got the booking. It's not like you're locking yourself into a whole month's stay at a place you may decide you don't like. If you stay in Patong for this trip you can get a feel for the place, see whether it appeals to you or if you like a different part of the island more, and next time you'll be in a better position to choose.
One thing you might consider, if you want to save some money on lodging, is keeping an eye on Ensogo for deals. Right now I don't see any deals for Phuket hotels, but sometimes there are some very posh places on offer for a fraction of the regular booking rate.
Roger that. THANKS for the tip M..