Compatibility for old Sram 7-speed shifter
Edit: Just checked an old copy of Sutherland's Handbook for Bicycle Mechanics (6th edition), and the SRT300i is actually Shimano-compatible, pulling 2:1 ratio. I've updated the comment below to correct that mistake.

Many SRAM shifters, are only compatible with SRAM "1:1 Actuation"* derailleurs. Shimano shifters and derailleurs use a "2:1" ratio of cable pull to actuation - the MRX you linked to at REI is one of the few SRAM models that are designed to be Shimano-compatible, like the SRT300i you have now.

(The SRAM ratio is actually closer to 1.1:1, while the Shimano ratio is about 1.7:1, but the common terminology rounds these values. The spacing between cogs in 7- and 8-speed systems is 5mm. In practice, this means that one click of a SRAM shifter pulls about 4.5mm of cable to move the shifter over 5mm, while each click of a Shimano-compatible shifter pulls only 2.9mm of cable to do the same.)