is the performance of an SSD that great in comparison with a mechanical HDD?

I'm undecided on whether to upgrade, I don't have the deepest pockets so would probs get something like a 250GB which is not great as I do need my space, however I am wanting a performance boost. I was wondering if I used my SATA HDD externally then used an SSD internally whether this would cater for my needs, I would use the SATA HDD (which is currently in my machine) to host my VMs on, could this setup work? do you think SSD gives that much of a performance boost? The only reason I...

is the performance of an SSD that great in comparison with a mechanical HDD?