I share this uncertainty, but it's really just a matter of screen size vs portability for me. When in the office it's no problem, I use an external monitor (the MacBook's own display is just somewhere I can stick video conferencing displays where they won't get in the way of my checking other things). But at home or when off-site the lack of space on the 13" screen is often an annoyance. Conversely the 15" would be better for those uses, but while it's much lighter and a bit smaller than my previous (2010) 15" it would be awkward on trains and very awkward on planes.Poki is right.
I am still unsure whether to get a 13" again or 15". It will all depend on the specs, but a quad core 13" is very tempting indeed. Portability with some power - I use all cores in a lot of apps I use so will benefit a lot on this upcoming upgrade.
Love the design and portability of the 13".
May have to get a MacBook and a 15" to remove the compromises.
Either will do for cpu power (any serious computing is done on the grid, not on a laptop), and hopefully storage options will be more reasonably priced in the next round. So for me it's purely a matter of which compromise on size vs display area will be least frustrating (I'm not going to waste time wishing for an intermediate size, because that's not going to happen).