what is the difference between gallery2 and gallery3, except the answer, one is more recent of other

Is there huge differences which can make the difference ?
do you have some warning or incompatibilities of gallery3 with sme8 ?
I started to take a look to this software with the idea to make a rpm, but the version available in epel for el5 is gallery2 and gallery3 is only available for el6.
For us, epel is an easy way to follow updates, but not all softwares are up to date