GSMArena.com - mobile phone reviews, news, specifications and more... is best site to compare phones in my opinion, just have two tabs side by side and check the differences, that's how I do it everytime I'm in doubt.
It's also a good idea to check reviews online and on Youtube. See people "play" with the phone you want and check if it's up to your standards and you feel like it's worth the money, nowadays phones from a few years ago are still amazing and they don't seem to age down. You probably don't need an S7 Edge 128GB when you only want to play basic games and text.