A 700c cassette wheel would be a nice upgrade, it would broaden tyre choices and give you a stronger rear hub with more gearing options.
A higher volume 700c like a 32 is going to give an equally plush ride.
On the other hand, if you like the way the bike is set up with 27's and you already have invested a little in tyres a simple swap is going to be pretty economical... the 610 came with fairly decent hubs so rebuilding the wheels around those is also a nice option if they are in good condition.