External SSD question(s)

Okay, I don't really keep up with disk drive stuff much any more. If I were to buy an external SSD which connector on the back of the pc will get the fastest throughput?

I know there is USB 2.0, USB 3.0, eSata, and probably a few others. Should I just buy an internal drive dock and go that route instead. I actually have a couple drives running via a StarTech.com and it seems to run fine. No idea if I'm gimping the speed of the ssd by using the dock though. I'm not sure if there is a hdd speed test app like there is for video.

I have a pretty decent rig, but heck if I can figure out how to add drives inside the case. It looks like it would hold 5-6 drives in there. It's a full tower for goodness sake So, external it is...

P.S. I have 5 drives going already. They're just drives I've pulled from other computers. I just keep moving them into the new boxes!

SSD - always internal. Dock, if needed, yes. Usually when you don't have a place to screw SSD on. My case has two places for SSDs but I can always add a dock, for example.

Don't buy USB external SSD - too expensive and not as fast as when plugged directly. Unless in consoles, those still use outdated SATA2 whereas USB 3.0 external SSD would be faster than internal. Moronic designs.

5 HDD drives you have toss in the nearest trashbin or try to ebay them. Instead of 5*500Gb mess, buy yourself a nice and shiny cheap 4Tb HDD. Internal, external, doesn't matter.

EDIT:
Rereading and now I have a feeling I misunderstood something... If I did, sorry.

While USB 3.0 will do 5gb/s (SATA3 is 6gb/s), that's max throughput, and doesn't account for file size, multi-threaded copies, bus efficiency, etc. Internal SATA3 will be much faster and more efficient.

You didn't mention usb 3.1 but if that is an option it'll perform decent for linear.
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If you want to connect the ssd internally buy a 2.5 to 3.5 converter (it will hold two drives) from silverstone. There is actually an altenartive model by some clone that uses clip instead of screws - but i prefer the silverstone.
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I have a feeling that if you are using a pile of old disks using esata or usb 3.1 will be fine for the ssd - not sure if you want this as a portable drive that gets moved around or whatever.
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Anyway if you just want to hook it up to your external connector go with esata but the only cavet is esata is dying and most new computers will have usb 3.1 (not sure how long usb 3.0 and usb 2.0 will last - maybe a while for stuff like keyboards).
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I would avoid esata because of the connectors it is the most likely to be obsolete in your life time. I guess part of the problem is technology is still rapidly evolving. I suppose at some point in time (not today) most drives will be thunderbird or usb x or some to be invented interface.