Ok so an SSD will access files a lot quicker than an HDD and is significantly lighter. Windows will boot very fast on an SSD about 10-15 secs.
However when you store files that are access many times on an ssd, the life of an ssd will decrease. They often have a life of about 10 years when used as a boot drive. However an SSD will not have a lot of the problems HDDs have (for example physical damage won't affect them as much), and they are more reliable than hdds
An SSD that is 128gb will usually be enough as a windows boot drive.
You might want to consider doing a setup where u use an ssd as a boot drive and an internal HDD as file storage. Its easier to manage than an external usb hdd, but its up to you. it can work with an...