The market for a given region/sector will be the determining factor in a/c choice. General things to consider also:
- will the market grow? and, are we trying to make it grow?
- how much infeed to the route structure can be expected?
- how many spares will be needed to meet possible MEL requirements? and where will they be kept?
- how many a/c do we need in our entire system? (nbr of a/c vs. nbr of seats)
- would commonality have an impact on a/c selection, for instance the A318 to A321 range?
- what happens when/if we expand?
Training and maintenance are high dollar items, with maintenance continuing to mount up over the a/c's life. jetBlue, for example, decided that yes, it is more advantageous to split between a/c types to feed one into another. Mind you, they went for some years with only the one a/c type before determining they desired some fleet diversity. Time and market share will usually drive any decision on fleet type. For niche markets, where smaller may be better, start small and work up. My 2 cents worth, backed by 26 years in this biz.
Cheers, y'all.