I would encourage you to at least read the basic rules for both games which can be found here:
You will see they are similar, but different. If you go with minis the WWI and WWII planes are in different scales as well. Both games play great, but there is currently more available for WWI.
IMO, WWI is better for battles with a small number of players. Even with one on one a dogfight is possible because the planes more slower and turn faster. WWII you need to have at least two on two to have a decent dogfight and even more is better. With both games a single person caqn easily control two planes.
My prefernce is WWII most because I have more familiarity with the planes and history. With that said this past week we played a few games of WWI after taking a long break from it and found it very fun.
Like I said before it is a sipplery slope, if you get one do not be surprised if you hav the desire to get the other in time.