DPI is resolution, sensitivity is primarily how fast the cursor moves. Higher DPI (dots per inch) means the cursor has a smoother, more accurate feel for pinpointing precise spots, as per in sniping. Higher DPI mice have more increments the LED or laser reads with, a denser scan so to speak. However going too high in DPI can have adverse effects on the sensitivity for things that don't require precise pinpointing.
For that reason if you have not tested the one you want to buy, or can at least return for refund with no hassle, it's best to have an on-the-fly DPI adjustment on the mouse somewhere, and not underneath. Somewhere that's easily reachable and adjustable while you're playing. That said, many settle on a given DPI spec that is either not adjustable, or leave it at one setting. Once you know what amount of DPI is comfortable for the gaming you do, it opens up a lot of brands, models and price ranges.
In other words, you don't necessarily HAVE to have on-the-fly DPI adjustability. It just takes the guesswork out if you're not sure how much DPI you need. In your case however you already own one, and it's even DPI adjustable. So now you just need to differentiate between speed of cursor movement, and smoothness of precise pinpointing, and find the sweet spot, or keep adjusting as you go.
To put it another way, ever heard of mouse look getting "squirrelly" when it's cranked up high? Fast cursor panning in and of itself if left to just the mouse options can be hard to control. Ideally you want speed and precision. To a degree it depends how the game you're playing is coded, esp games that are made on console and ported to PC. That in itself is one of the best reasons to have an adjustable DPI mouse.