AverageBoss

Unless you are going multiple sizes in either direction, any difference is marginal at best. While smaller bases allow for one or two extra models to attack in close combat, larger bases provide more area denial, enhance the power of aura effects, and give you more options with anything that requires measuring range from the base. They went to 25 round at AoS launch, but the boxes as of DoT now come with 32s. Ask your local TO, but at the end of the day, you should be fine using whatever the models were packaged with.

Some LotR models are fantastic, and look much better than WH/AoS models you would be replacing. LotR eagles make great HE eagles. Ringwraiths make cool and varied wraiths (and they were actually bigger than the metal VC wraiths that existed at the time). Though that was better back when wraiths were units and not just characters. LotR bats look tons better than the actual VC bats. A fellbeast makes a good wyvern or dark dragon.