Most ext. HDs use int. type HDs as the storage media. Small and large sizes to include SATA or EIDE types. So, if you have an older int. HD laying around, purchase an ext. USB case and/or use a new HD instead. An ext. HD already made for use tends to have some warranty protection or extended for added cost. So, that's a good deal for any user build ext. HD will only cover the HD itself, the ext. case being a separate item.
If you like look into "HD dock/socket" type mounting. These can use int. or old/new HDs and treat the HD as if like huge flash memory. These too come in 2.5/3.5 size for SATA use and are a quick cure if you use many HDs.
I don't see any big advantages other than current int. HDs is buying one with 3-5yr. warranty can be had vs. a typical 1yr. one. Also, as build ext. HDs usually have some s/w bundle(kit) or provide ready to use setup vs. build and format and access, etc.. Also, all drives above 3Tb MUST have some fix for use on XP or any system limited by over 3Tb access.
FYI- Ext. HD do fail, so back-up multiple times or save super critical data on discs. Treat the ext. HDs like eggs and mount/dismount properly. if it fails and covered by warranty, unless so stated, data is not covered(retrieved).
Just as the title says, which one would be better in your opinion in terms of best value for money? Purchasing a USB HDD or a fast internal HDD and use it in a HDD enclosure? This will be used mainly for data backup/restoration purposes so my main concern is the speed and longer lasting product. I am looking at 500GB-2TB but depends on what you recommend and how much it costs.