Design differences. The 410 is an old brute force design from the days of ignorance to comb filtering and loss of mid dispersion.
I'm with you on the $50 no brainer, but at the same time, 2 rumble 210s would outperform a 410, they hit harder in the lows than a single 410, have more cabinet volume per driver, and the staggered design helps dispersion.
410 is not any different than any typical 410, but the 210 is part of the crop of new vertical staggered large box 210 (compared to say a traditional SWR/GK style 210).
And did I mention the form factor and how well it matches the combos?
2 210s is a lot easier to manage than a single 410 once you get older.