The comparison is hard to make. Apart from what they sound like there are decisions to be made.
First of all the expansion board isn't just a rhodes/wurlitzer emulator. it also contains some mellotron samples and most important of all a bulk of Hammond B3's.
So a fairer comparison would be to oppose EVP88 + Native instrument's B4 (hammond softsynth) and the 60s and 70s expansion board.
Secondly, the expansion board is way better by means of mobility... do you want to carry around a computer all the time or don't you perform live and is this no problem to you?
Thirdly, the waves of the expansion board are editable in patches. this means that an expansion board is not just an upgrade itself but is also an upgrade to the other expansion boards in you synth and the jv/xp soundsource. you can combine those waveforms with many other waveforms already present in the machine and have lots of fun with that.
Fourthly, because the exp board is in your synth it'll always have a different feel then a softsynth. no matter how low your latency is. you can just feel the sound coming from your jv/xp itself.
fifthly and lastly. with an expansion board you buy waveforms that can be tweaked and all. but some things are simply in the wave and cannot be altered. A vibrato, tremolo, or leslie effect in the sample can't be removed from it. one can off course allways use a clean sample and apply the effects from the JV/XP engine but those just don't work like the old machines (rhodes/hammonds/vintage synths) responded. that's why the waveforms on the expansion board already encorporated many effects. And to an even further extend the options of the board are limited. If one looks at the options of a patch. there are just 4 combinable waveforms yet the variables in a proper drawbar organ with leslie and amplifier are way more then you can ever stuff into those 4 tones plus some effects and filters. Softsynths try to do this more succesfully. think about this: how can I regulate, the speed of the leslie of bass and treble seperatly & change the character of the key klick & open and close different drawbars and have a different sound for footpedals, lower manual and upper manual & change the character of the distortion in the amplifier of the organ & adjust the swell option on a hammond all from my jv/xp sound engine at the same time? (sound architecturally of course, nobody has 3 hands).
softsynths like the EVP88 and B4 can perform many more of those tasks + come really close to the actual sound then an EXP board could ever do.