Hi Smitty!
I believe the plus has the addition of the EQ (slide) adjustments and the standard 50 cal. just has the basic tone (dial) controls.
Also, if you're interested, go on Mesa's product "manuals" they have the 50 cal. and 50 cal. + listed. Your plus is just a little more versital as far as be able to "dial in" your tone. Otherwise they look the same to me.