After having an SSD at home, my work computer, which is no slouch (a Core i7-870 with 12GB of RAM) feels downright sluggish. Don't get me wrong...in computing tasks it's fine, but it seems like it takes forever to boot, and forever to load large applications. A fast SSD is truly an amazing experience, frankly, and well worth it. As a whole, it just makes your computer feel so much more responsive. It's a bigger jump in usable speed for your machine than two generations of CPU upgrades. I don't think you'd see any appreciable difference in day to day speed from a 128GB SSD to a 256GB SSD, though the 256 would be faster, but at that point, it's like asking which is faster, a fired bullet from a pistol, or one from a rifle. Sure, the rifle round will almost certainly have a higher velocity, but how fast they hit their target after pulling the trigger is essentially the same in perception.
So, if a 128GB modern SSD is a pistol bullet, and a 256 GB modern SSD is a rifle bullet, in this analogy, a fast HDD is like driving a sports car to the target. Sure it gets you there fine, but it sure isn't a bullet.