Which rank is senior in the Army, Master Sergeant or First Sergeant?

I'm sort of confused by your questions but I think what you're asking is which rank is higher than the other. So both Master Sergeant and First Sergeant are the same pay grade which is E8, however First Sergeant is senior to and outranks Master Sergeant. You can tell the difference between the two being that a Master Sergeants rank has 3 Chevrons and 3 Rockers, the First Sergeants is the same but with a Diamond in the center. The First Sergeant is the Senior Non-Commissioned Officer of a Company and assists the Company Commander which is usually a Captain.

A Master Sergeant usually assists or is in charge of an office at the Battalion or Brigade level, sometimes a Master Sergeant will be in charge of a Detachment or a School. A Detachment is smaller than a Company. A School is literally that, a place designated where Soldiers conduct designated training.

EDIT

Didn't see that SGM Dawson beat me to the punch, his explanation is good follow that.

Both are an E8 pay grade. First Sergeant is a "Command" position, working for a Company commander. There is normally 4 platoons in a Company plus a Headquarters section. The 1SG is the Senior NCO of the Company, the CDR's right hand in keeping him in touch with the enlisted/NCOs of the company.
A Master Sergeant is in a position of authority, usually a section leader, shift leader in a Headquarters or similar type of unit, usually a Non Commissioned Officer in Charge(NCOIC) of a section. The MSG position is usually found in Battalion or Brigade level staffs, or sometime there are units which are not big enough to be a company, they are called detachments. Detachments are not usually rated to have a First Sergeant but a Master Sergeant. When in this position, the MSG is usually sent to 1SG school as the MSG in addition to his unit duties will perform the 1SG duties as well.

Neither they are the same. One may have a position over the other but it can go either way.