for extra laptop storage
What you’re looking to get out of your hard drive should answer this question. The primary question I think is this: what’s more important, portability or price?
If you want something that can still potentially fit in your pocket, go with a 2.5", hands down. I’ve used one for over 5 years now (120GB lol) and it’s been extremely useful for backing up data as I primarily use a laptop. I’m fairly sure they’ve hit TB even in this form factor, but the price will definitely be higher than that of an external hard drive.
As for external hard drives, they are certain to be cheaper than the 2.5" ones for the same size, but I believe they require you to carry a power cable. I don’t know if there are new ones out there that don’t require an external AC source, but the ones that I’ve seen do. We keep two of these in the house with the desktops for backup purposes.
Ultimately, I think it comes down to how you’ll use this hard drive. If you want to bring it with you wherever you go, get a 2.5" one. If you want a beast of a hard drive to store everything under the sun (or a lot of HD videos), I’d suggest an external hard drive. The 2.5" drives will be slightly more expensive, but they are certainly worth it in terms of portability.
Hope that helps!
Just an afterthought… depending on how you use your drives, it may not be worth your while investing in the 1TB drives. Since I’m not a traveller away from home for long periods of time, I prefer to have many smaller drives rather than 1 big drive. For daily use, I usually carry a 16GB memory stick instead of a hard drive. That way if I lose it, it’s significantly cheaper to replace and less data is lost. If I need more than that, I take a 60GB 2.5" drive before the bigger ones for the same reason. It’s convenient having 1TB with you at all times, but it’s also a huge hassle if anything happens to it.