Islander is very good. You won’t be disappointed 
Also I had the same question about the difference between the 310R (which I already owned) and the 414AW. I got “persuaded” to get the 414 too (and as it has ended up at the top of my charts thread, I thought it deserved my purchase. That’s my excuse anyway and I’m sticking to it!) and it is actually definitely different enough in character and feel to the 310 to warrant having both - especially if as you say you have a fondness for Twins.
Goose is another great Twin (my top plane atm).
Can’t comment on the 737’s but grab those two and a 737 and that’s still within your monthly budget, right?! 
I could spend a lot more of your money for you, if you want!!
Ryan ST-A Special is very cool. The new Hellcat that released today?
And it needn’t be Aircraft in your budget ?!
How about general enhancement mods!?