You should probably try to research rank 1-2 for US and USSR first before deciding to buy, shouldn't be too hard to research all of rank 1 over the course of a week or two. It gives you an idea about how each nation plays for ARB props at least. They should all play relatively the same after rank 1/2 just slightly faster with more power and better firepower. That is if you are going to play props ARB which I do recommend, it's a good change of pace.
For Jets the US really only has good rank 6 and rank 8 jets in the tech tree like the Sabres, F-84s, F-80s, F2H, and obviously the F-16 and F-14.
For the USSR you have the La-15, La-200, Mig-9, 15, 21SMT, 23ML, and 29 that are good.
You might want to watch a few videos on which planes you are interested to fly for each nation. They fly differently and come with different play styles so knowing what you are getting into might be a good idea. from my experience, the USSR provides the best planes for low experience players since they perform best at low altitude and are pretty much jack of all trades in terms of performance, they turn relatively well, climb okay, are moderately fast, and have decent guns minus the ammo count. USSR also has the most planes that are good for their BR per rank.
But just getting the Su-25K or A-10 to grind to jets as your main goal might burn you out pretty fast so I recommend at least trying props, it gives you something else to do instead of grind 24/7. If you are just constantly looking at what is next in line to research you will definitely grow bored of War Thunder.