Guderian or Montgomery, which is better

Usually you don't just buy generals because you can do so. You buy them because you really need them for specific purposes. You won't have enough medals to purchase every general you like, so you have to set priorities. You should have at least one (better two) top tier tank general, one top tier Artillery general, one Admiral and one powerful Infantry general on each side. The rest is optional.

Don't waste your medals on mediocre and even good generals. And buy generals only if you have a clear idea how to use them. Try to find out where are your weak spots, and how it is possible to improve your overall performance. In my experience buying a top tier tank general is a huge step forward. On Axis side there are only two really good options: Rommel and Guderian. Purchasing either one of them is a good choice. I recommend you to purchase both eventually anyway. There are four good tank generals on the side of the Allies. Everyone of them has his pros and cons. And Monty is one of the four. So buying him is not a blunder either. But you should have a clear plan what to do with your generals, otherwise you will reach your plateau sooner or later. So, don't do any purchase in a vacuum.