also should I get a USB3 SSD or TB2 SSD?
I think the TB2 drive would be faster, but a lot more money. I am sure you would see a very noticeable difference in load speeds with the USB3 drive.
Yes, TB2 will be faster, but those drives are too pricey for me.
I don't want to boot from it though.
I am sure you have your reasons for not wanting to boot from the external drive, but you could always have an external boot drive, and use your internal HDD as a backup. You could back it up every day using CCC or some other software.
I don't use external boot drives anymore, but I used to a lot. I remember making the free space of an iPod Mini into a boot drive for trouble shooting issues.