It is easy enough to try out the difference between the editor presentation types. Configure the editor presentation one way, create a work item, change the presentation kind and create a new one (takes about a minute). 

The difference between

1. Kind Text and
2. Kind String 

is that

1. Text kind is for an multi line entry
2. String is just one line to enter

With respect to

Also, is there a "length" difference?

A presentation is a presentation. It only presents something in some way. The data is finally stored in a relational database and current relational database systems basically have length limitations for columns.

So the obvious answer is that there is a length limitation (different for small, medium,.... string) and changing the presentation does not change the limitation.