How many GB of RAM are you using now? Does that run smoothly? if it doesn't, is it because of the RAM or CPU?
The last one clearly has more RAM, but if you are having problems with the CPU, that may not be too big of an issue to get one of the servers that has less RAM.
If you did want to run one of the servers that had less power though, you could always look into optimizing it (Spigot like you said would be a good option), and anything with a database option, you should look into using it. Also, many plugins seem to autosave, but if you can get one that saves all of your data at specific times, that should be fine.
This may not be the best help you'll get, although it should be a start.