The cost per seat is usually lower for bigger birds. I say usually, because when you buy a 737-600 for 110M$, while the A158 is at 17M$(had both fleet groups in previous GW3, and yes, it was to that level), when you count the owning cost, the benefit of size is not obvious, and if you lease, you're screwed.
"It depends". I've been taught that iw was tha standard engineer's answer. I like it a lot. It really depends on your strategy, on the game dynamic, and on your specific opposition. For example, if you have a lot of money, and an opponent you want to kill on 280NM routes, you can drown him under SW3. Works only if he is weak, and if he does not have a lot lf longer range(pro-tip : don't count upon it).
Your strategy is an important thing. The strategy is usually a choice between good routes and not much opposition. 737/320/919 are planes who are kickass on many of the most juicy routes. Of course, those are routes with a lot of opposition. In current GW3, I just landed 3 more 737 lines from Edinburgh to Amsterdam. But my strategy usually goes to poorer routes...with less opposition. Like a SW3 flying from Edinburgh to Kangerlussuaq, with a tech stop in Keflavik. I don't risk any opposition here. I don't risk big profit either, so I have also more risky routes, like the ones to Amsterdam.
The game dynamic is important. In previous GW3, CRJs were going to an insane price. I replaced them with A148 who were half the price for a better capacity. They were so cheap that I could by them in great numbers, while for the same growth level with CRJs, I shall have leased most of them. And profit would have gone down. But it was specific to that GW. Be attentive to price dynamics, and unlikely bu juicy routes. One A158 Flying 8 hours a day was above 1.6M$ pretax profit per week. I bought that one around 17M$. Who says better? I probably would have had a slightly better profit with a 737, but growth was easier with that one. 737 were scarce on the used market, horribly costly new, and 4 years to wait new. I had my A158 within 4 months. But, of course, this is not always true. Analysis of circumstances is important. And if it's year 2020, and 737 & 320 are still impossible to get, look for 919. Within their range limits, they are very decent, and often easy to get. (but it's a fleet group in itself, so not an easy decision).
Opposition, in current GW3, pushed me to downgrade a few 737 routes to E175. Not much, but it happens. It's not fun. But better use those 737 for juicier routes. That's situational, as always, know your routes, know your familiy groups, and depending on that, make decisions.
EDIT : I forgot : slot availability is also to count. If Heathrow was slot-unlimited, I wouldn't fly 737 form Edinburgh here, with all the opposition. But at this slot cost & scarcity, better do like everyone, and go for true single-aisle birds.