About slanted vs. straight cabs: with a mic in front, there will be little to no audible difference. If you're playing live, a slant cab will project more sound up toward you, rather than blasting your knees. This is where the difference in tone comes from- it's like on- vs. off-axis micing. A slant cab will make it easier to hear yourself on stage. A straight cab will get more lows going in the room. Take your pick.
This is kinda true and kinda not. Since the highs are pointed at your head (from the top 2 slanted speakers), you get the sense that there are more highs, and thus a straight produces more lows in the room. that's not true. Get your head inline with those speakers in the straight cab (i.e. bend down and take a listen) and it's the same effect as a slanted pointed at your head. Now, if you are on a raised stage those speakers will sound to the audience like a slanted would to you on stage (or closer to it). The idea that a straight "produces" more bass is wrong, IMO. The only argument for it would be the increased mass of a straight contributing to cab resonance and the idea of it being more boxy...Heck, Mesa oversized "straight" cabs still had slanted baffles...
IMO, the biggest difference will be projection since the baffle isn't slanted. The straight will be more focused projecting out rather than having an increased spread from the angled top speakers.
The original idea for a slanted cab was for bands to have full stacks and that top cab to project far enough to allow the whole auditorium or stadium to hear the guitars well...The PA's in the 60's just weren't that great, so they had to rely on stage volume and projection to get the sound to the audience in large venues.
As far as speakers go, V30's are known as having a slight high mid spike, good for cutting the mix. T75's are known as having a more relaxed mid and a good amount of bass and highs...This is why they are a good combo with Marshalls where they can tame the "Marshall mids" - also why some people call them fizzy.
FWIW, I believe Mesa coats the cones of their V30's, so they are slightly modded from regular V30's. Also, Mesa rates the Celestion V30's in their cabs at 70 watts a piece, while Celestion rates them at 60 watts.
Cab construction definitely plays a role in the sound as well. The number of plies, the type of wood, etc. MDF cabs tend to be stiffer and more sterile sounding with a good amount of bass, as the dense MDF lowers the resonant frequency. The stuff cab also allows the "speaker tone" to be more pronounced because the play between the speaker and cab is lessened due to the density of the MDF.
A nice quality ply cab made from good wood tends to sound more lively and resonant better, IMO. Also, terms like void free or marine grade can also indicate that there is less knotting (thus less filler) used which tends to mean it's a better quality wood. Baltic birch seems to be heard a lot in the construction of nice quality cabs, at least in my experience.