Another factor is if you needed a little more capacity in the bank with Li it came out in an earlier thread you should not mix say a 200 with a 100 like you can with "ordinary" batteries.
So the 250AH size in one batt might be useful where if you did have a 200 already but needed another 50, you could not get a 100, but would have to get another 200. So be sure to do your your sums first and get the 250 instead of the 200.
You really need to know what your draws will be, which few people have any clue about estimating. I suppose asking about costs for Li is the same as other things---if you have to ask, you can't afford it!