I think it's great you're talking about it, and it's good to have a theoretical plan. IMO if you both want something, it makes sense to (plan to) share equally.
DP and I had about a million conversations about this. Now it's actually coming due (Sep!) we are planning to both be 'off' (sabbatical/mat leave) for the first 3 months, then share the parental leave out (you can flip-flop a bunch of times, so could as teachers you could alternate by term, if your employer is feeling tolerant). After that, we'll see: we are an industry where short term contracts abound so we make keep alternating in 3-6 month chunks or we may both go permie-but-part-time or some other combination. Or have a second and start the loop again, or be sick of it, park Bean in an 8-to-6 nursery and take high-powered jobs in London... The options really are endless if you're willing to jack in the idea of following the standard career path of get job do job 9-5 for 30 years retire.
Personally I don't think who makes the most should be a factor at all, in an ideal world. In the real world, it still shouldn't be the only factor, and focusing on it risks compounding the problem. In this case, would your OH be earning as much as he is now if he'd taken 3 years out at your stage? I really doubt it. Being better established can often let you get a better leave package (as you have the seniority to negotiate) and make it easier to return as your skills are more in demand.
It's worth thinking about the impact on your careers - both of not going back for 3 years, and of going part time / contracting as this often means stepping down to get a flexible position. DP looked startled when I asked what he'd give up to be the SAHP (living standards and career both take a hit) - it genuinely hadn't occurred to him that would be an issue as 'having it all' is the default for men, not the super-human ideal.
Another (theoretical) spanner in the works is DC#2, #3 etc - if you go the classic SAHM route and want to stay out of the workforce until your youngest is 3, this could easily be 5+ years, and 10+ isn't uncommon if you have 3. That's a long time to rely on one person doing a particular job, and risks tying your family to crappy circumstances (a job that goes sour, a house you can't afford to leave, etc).