Ryzen 5 3700/3700X at the same clockspeed is 15% faster across the board, due to 15% single core/single threaded IPC (Instructions Per Clock Cycle) improvements. The 7Nano-Meter manufacturing process makes it more efficient, using less power and outputting less heat, which in turn allows for a higher theoretical maximum potential - The cooler the chip runs, the faster it can clock, the faster it clocks the faster/more work in a single percievable unit of time it is. Memory clock speeds are a bit higher (although my personal RAM kits all run at the same 3200mhz so this does depend on your RAM as to weather this really matters or not)
If you was to overclock the 3700/X, your power consumption will go up, and it's possible to shatter that 65 watt limit.
The additional price is due to the sheer speed advantage the 3rd gen chip has over the 2nd gen chip.
For 90% of people, either would make an awesome CPU however if you are playing games or emulation know a few things
1. The 3000 chip will give you higher minimum framerates, which is important for competitive stuff.
2. The 3000 chip will give you a higher maximum framerate.
3. If your monitor is only 60hz they will both do just fine.
4. If you plan on keeping the parts for multiple years or plan on playing games from the PS5 generation then the 3000 chip is what the new Playstation and Xbox are using, so that one would be preffered over the 2000 chip. However it will be likely 3 years before games are released that require that much power.
5. If you have the extra coin, go with a 5000 series chip - They are (depending on what you're doing) between 13% and 21% faster, and the 5000 series is the series that dethroned Intel in everything, Zen2 (3000 series) dethroned them in several use cases but the 5000 series took EVERY crown, including the gaming crown, and it will be the last Upgrade path for Socket AM4, the 6000 series will be AM5 featuring DDR5 and PCI-Express 5.0, but that's not likely to land untill Q4 2022.
I hope this helps you! You can find more comprehensive information on Youtube at GamersNexus, or LinusTechTips. Linus tends to be more to the point with comedy thrown in, while GN tends to be one of if not the most deeply analytical in their testing.
(EDIT: - Have a great day & stay safe out there!)