An SSD has a faster read and write-speed compared to HDDs. However, if you're looking to use a drive for years, you should rather opt for an HDD. Reason is, that SSDs wear out over time and are definitely not made to last years on end. Additionally, HDDs give you an idea when their demise is inbound, because they make nasty noises when the time has come, giving you the opportunity to save and backup your files. SSDs don't care about your feelings and just die at some point.
If the money aspect is not your greatest worry though, going for an SSD will improve the speed with which you operate due to the higher transfer-speed. In your case, I would go for a 1-2 tb SSD on which you run your os and keep files that you're working on right now. Additionally, I'd buy like 5-10 tb as an HDD to keep the files I'm done with or didn't start yet.