I disagree with this statement. If you've never been boating before, you have no idea what you want in your second boat, you barely think you know what you want in your first boat. It gives little to no good advice other than perhaps insinuating that the majority of people go larger on the second one. A better saying would, IMO, be....."You rarely hear people say 'I wish I had a smaller boat' ", or "You rarely hear people say 'I wish I got the smaller motor' ".
We're on our second boat, and it's actually the same length as our first. The big changes for us were propulsion system, fuel injection, lower maintenance, and a generally improved layout. We had ZERO idea that any of those things were even remotely on the list of items to consider when we bought the first one. We're still not even sure what we want for a 3rd boat. I like the idea of a 212X (for the twin 1.8's in a 21ft), but I would have to raise the header in the garage, or store offsite. If we're storing offsite, might as well get the larger 242SE (large bimini and E-Throttles, I don't want the wake stuff really). But this is more of a storage consideration than anything.
@Grown Azz Kid I would probably get the 210 in your case. I like the 1.8L engines over the smaller MR1's. Not quite as many revs, but similar real world performance. I kinda like the look of that 210 over the older 230's as well. Completely personal preference there, I think either will suit your wants just fine.