Movement speed changes are only noticeable after med, so that's not a problem
The shield bash is stronger and looks better
They're just as resistant as any other 5* shield, but a more unique look
You can get one to be prismatic if you have one of those 4 personal colors
And you can get the shells for all 4 during a single event(this past event alone I got the shells for all 8 5*s and a 3* with a couple left over)
I would say keep it
Compared to other shields, tortoshields have extra shield-bashing power, but you incur a movement speed decrease.
If you shield-bash a lot, then it might be worth it. If you value movement speed a lot, as I do, then it's not worth it.