The main functional differences are that the + has built-in YM chip and it uses an upgraded micro-controller which gives extra firmware storage space and work RAM. The addition of the serial IO might be a draw to some, its potential for MIDI and WIFI are presently being assessed.
In terms of build, obsolete through hole chips have gone. The twin GALs have replaced by a single CPLD. The RAM chip has been replaced by a teeny wee SMD part. This (at least on paper) represents a fair reduction in power requirements, though to be honest I haven't measured
Overall the boards are very similar functionally, but the + benefits from a new 'ready' signal which obviates the need for chunky timing loops in the ROM and produces a modest speed increase as a bonus. The ROM still busy waits for a command to complete but can now respond a scant few microseconds after the result becomes available. A small win but it accumulates quickly.
Most of the changes are firmware oriented. The ROM is changed quite significantly, though all entry points to the standard ROM routines are unchanged from the ZXpand Classic version - compatibility was one of my highest priorities. The changes have centred around size reduction and allowed new code to be added to assist in the use of 'streaming' blocks of data quickly from the sd card.
Some features have already been back-ported to the classic, namely the serial LOAD functionality and faster data transfer potential that I first developed for Dragon's Lair. I say potential because changes to the classic ROM would be required to fully exploit this feature on a day-to-day basis, but user code can take advantage of it. I suppose this is where the red line exists for the back-porting of features - if a change to the ROM is required then the feature stays as plus only.
So in all honesty if you have a fully loaded ZXpand+AY then you won't benefit from splashing cash, unless you want a more compact arrangement on your desk.
Chips tend to operate within some defined range of limits, and sometimes chips are at the edge, other times in the middle. I have seen ZXpands that failed to work until I swapped chips around with others, it was possible that a set of chips would just not work together yet when part of a different set were fine.