My copy of 5th is buried in a box somewhere, but from memory, on the tabletop, largely they're the same - psychology works differently as there are different stats for different tests (Cool, Willpower, Intelligence as well as Leadership) - also the effects might not be quite the same, and you need to change tactics to take full advantage. Combat is slightly slower as the to-hit table is a tad tougher (more rounds for manoeuvring), magic is very different (but almost exactly the same as 3rd).
Off the table, the biggest difference is there are no army-books, you can use Ravening Hordes if you want Warhammer armies, but stylistically some feel a bit different, and there's Nippon and stuff that don't appear in 5th (zoats? fimir? wood elf shapechangers?). The biggest advantage is the points sytem is more detailed, so you can design troops from the ground up, and mix and match your forces as you like, Elf & Dwarf alliances against Orc Necromancers and all that good stuff.
