Matt welcome to the show!
This is a really good question. The first thing I would to is look at your current/favourite bat and see where the most wear on the bat blade - this will give you the best indication. If the most wear is near the bottom, then a lower middle and higher up the bat a higher middle.
The next thing to do is consider what type of putches you usually play on - slow and low = low middle fast and bouncy = high middle.
Next you should think about the standard of cricket you play, if you are playing against tall quick bowlers, then a higher middle will help, but if you are facing slow or medium trundlers - which is most cricket a mid to low middle will be better.
Finally where the middle on the bat is will have an impact on the pick up of the bat. So be aware that typically a low middle bat wont feel in the shop as nice as a high middled bat, but don't be put off by that.
Usually in England I would suggest you should go for a middle to lower middled bat, because of the pitches we have.
Also while I am on that subject, when choosing a new bat you should typically try to do this with your gloves on so you get a good feel for the bat in your hands as you would in the middle.
Hope that helps