I plan on Buying another Cabinet, to run with my current one in parallel, any suggestions?

I am not sure if I should just buy the exact same cabinet loaded the same, or a different one my current is a peavey 5150 slant cab loaded with vintage 30s
was thinking of getting a straight mesa 4x12 cabinet, I have heard it is best to have the same size speakers in both cabinets. If its not a big deal a 2x15 mesa bass cabinet might be nice.
I'm not sure about the whole same speaker size deal - I've never heard anything about that. I know bass player's mix-match their speaker sizes (normally one 4x10 cab and a 1x15 cab).

As far as to which cab you should get, I would suggest a Mesa straight cab or a Randall straight cab - depending on whether or not mix-matching speaker size is a problem, then getting a 4x12 or a 2x15 is totally up to you.
my other guitarist has a custom madison (we are endorsed my madison) 4x12 and a straight custom madison with a 15 and either 2 10's or 2 12's.
its anaweseome setup.
Fender MIA Standard Strat
ESP LTD Viper 407
Epiphone SG
Starcaster by Fender(modded with Bill Lawrence l-500-xl)
PEAVEY 6505+ (FJA transistor modd)
Peavey 65 watt transtube studio-112
Marshall 1960AV
Vader 2x12
ISP Decimator
MXR 10 band EQ
15s honestly dont add that much low end...if they did you'd see many more guitarists using them.

my buddy has a randall 2x12 1x15 and the low end from a g12-75 marshall 1960a destroys it
Baron K2 SE 120
MILLS 4x12 Afterburner
Eventide TimeFactor Delays
ISP Pro Rack G Noise Suppressor
BKP Warpig pickups