A very difficult question. Both Lorries & 4x4 are expensive to maintain but I think one of the main deciding factors is are you going to use the 4x4 as your main commute/go everywhere vehicle? If so & you do alot of miles you may find it extremely expensive on fuel & insurance. The excise licence will also be more expensive on the 4x4 than the lorry.
Depending on your personal circumstances you may find the cheapest is to have a lorry for transporting the horse & having a fuel efficient small car to use for every day use. This is what I'm considering at the moment.
I drive a merc 190e and I was going to sell it and buy a 4 x 4 to pull the crappy Rice trailer I have ( it still neeeds servicing and new rubber matting fitted)
Having thought long and hard my final deciding factors were as follows.
1. Fue and excise duty for a 4 x 4 were going to be expensive
2. I'm ex army and i hated towing trailers there. I would eel much safer driving a lorry.
3. Even the most basic ff lorys seem to have some kind off dayliving where you can shelter from the elements.
4. i need to stay over anywhere a lorry is a better option for kipping than a trailer.