I can speak from experience here as I have owned the Mesa 410, 210, and 115 cabs at the same time and compared them all. I actually had a pair of 410 cabs and a pair of 115's and I have tried them in various combinations.
The 410 stacked on top of a 115 was the worst pairing, mainly because the cabs cancel each other out in certain frequencies. Each cab sounds much better on its own.
The 210 and 115 stack is much better balanced but still masks some of the characteristics of each cab when paired together.
The 115 stacked with another 115 is downright shocking in how massive and full it sounds. Adding the second cab seems to make things sound 10 times bigger. The cabs couple together and it's like a freight train of big round tone. I really like that setup.
Of course the pair of 410 cabs worked really well but now we are looking at 200 pounds of speaker and not too many of us want or need that type of heavy lifting.
The 410 with the 210 on top was actually a big surprise and sounded a lot better then I expected. Keep in mind, Mesa uses drivers with double the power rating in their 210 so each cab is rated at 600 watts handling capacity. It's still a mismatch but this pair does sound really good together. I also found that the Mesa 210 sounded sweeter then the 410. A pair of them would be a killer setup although they are heavy and bulky for 210 cabs.
As someone who has owned a ton of high end gear, I ended up keeping my Mesa PH 412 for the bigger stuff and run a pair of PH112 cabs for everything else. They are the secret weapons in the Mesa lineup.
So for the original poster, I would jump at both the 410 and the 115 but would likely then re sell the 410 as they are easy to sell and find another matching 115. Or just keep them and both and use them separately depending on the gig and tone you are looking for. Either one of those cabs on its own will cover just about any gig.
Great info...thanks ! I actually have a line on a Mesa PH210 locally as well. So I have the option of a 410, 115 and 210. If you think the 210 and 410 pairing works really well, I may actually go that route.
And you found no "power" issues with tow 8 ohms cabs..and therfore too much power to the 210... as described in previous posts with the 210 on top of the 410 ?