From what I see, GW4 is currently in 2014. In this capacity range, you can go either turboprop, or regional jet. I tend to favor regional jets, as they can cover more distance for the same value, but of course the fuel costs are not the same either.
The purchasing price heavily depends on other players behaviour. I've seen the SSJ very costly, or very cheap, depending on the game. Sold thousands in the first case, just a few hundred in the second case.
The ATR family is a good choice. Though beware, the hi-density ATR42 has a different pricing capability than the medium-density ATR-72
The A148/158 family is dirt cheap, and does not drink more fuel than CRJs. They are, however, costlier to maintain.
The Bombardier CRJ is a very comprehensive family, from 50 to 100 seats. However, this versatility comes with a price. Same remark for seating & pricing than the ATR
The Q400 has an incredible fuel efficiency, especially according to its speed. It is plagued, however, by a very small family.
The Comac ARJ21 drinks too much.
The DH8 are cheap to operate, but suffer from a limited family and low speed.
The Ejets kick ass, but they are often costly, depending on the game dynamics.
The ERJs are really nice little machines, but, hey, they are little.
The J528/728/928 are cheaper versions of Ejets, with less choice, and slightly smaller. As they are less popular than Ejets, they often are a good choice for their lower price.
The F70 is a drunkard. The F100 kicks ass, and the F120 is even better. However, they are probably too big for your purposes.
The MRJ is a diamond, just be sure it's not too costly for your needs.
The Saab 2000 is an excellent machine, a smaller kind of Q400. Same qualities, same drawbacks. Beware the seating, though - it's 58 hi-density for the 68 hi-density of the Q400.
The Sukhoi Superjet has incredibly long legs for a regional jet. It is often cheap to buy. Somewhere between the J728 and the A148, with longer range.
As I'm not player there, I cannot check the relative prices. I'd say that if MRJs are below 40M$ to purchase, it's a no-brainer. But I've seen them in the 60s, while I could buy A148 for 14M$.....I bought 600+ A148/158, that game. So check all prices, and see what seem affordable. Make an analysis of your routes. Are they all so small and long that the ERJs are worth it? Are they so big that you can afford F100/120? Do you need much range, but medium capacity, like the SSJ? Do you want to buy them all, so that the cheap A148 is the answer?
And above all, is the MRJ affordable? If you don't need range above 1900NM, and 70/90 seats is a sweet spot, it's an incredibly good machine. Most players(me included) bypass them because it's too late for them. Remember it's the only regional jet of the 2010s implemented, all the rest, even the SSJ is from the previous decade. For the turboprop, only the new gen Q400 is really recent. Those machines really have a technological edge.
Last thing : remember that a machine that drinks 1400/hour and flies at 480Ktas does not drink more per NM than a machine that drinks 700/hour, but flies 240 ktas. That's the reason why Q400 & S2000 are so popular in the late game(with insane fuel prices) despite their tiny family. The MRJ is still superior IMHO, but don't overlook them.