TENS Machines

I can't answer your question as to weither to buy or hire but I thought my experience of using one might be a help anyways.
I had a TENS machine that I used for a knee injury a while back so use this. It was somewhat helpful in early labour but out of my three day labour I only used it for say 2hours tops. THe wires annoyed the crap out of me and the electrodes kept fulling off.
I imagine that weither they are cheaper or expensive they would still do the same thing its just that one may have more patterned charges and might come with a little more info about placement.
Placement is everything when it comes to TENS so make sure you do your research on that too.

Hope that helps.

I loved it for my sciatica, I didn't use it for labour though.

My personal opinion is they are a waste of time/money. I used the machine at home when my contractions were about 5-6 minutes apart and it drove me crazy! The weird tingly feeling was just the most annoying thing, I just couldnt understand how it would help. I too ripped the thing off after 10 minutes. I really wanted to like it, because I wanted a drug-free birth but I managed to have one anyway...and I am yet to talk to anyone who found the tens very helpful in labour!

The best relief was the birthing pool at the hospital!