Mobile wifi Device (Rent?)

Is there a difference between renting pocket wifi device before arriving and japan and at the airport in japan?

Are there different speeds and pricing.

Not sure if to rent in once I arrive or to book ahead?

Is there a difference between renting pocket wifi device before arriving and japan and at the airport in japan?

The devices and services are the same, but by renting ahead you guarantee availability and some outlets give early booking discounts. It also gives you time to shop around for service and price rather than be restricted to the few models available on site. You'll likely save money that way.

Are there different speeds and pricing.

Generally yes, but it depends on the carrier.

Not sure if to rent in once I arrive or to book ahead?

If you are positive you want to rent one then booking ahead is the safe bet. Here are some popular services to get you started:

http://www.pupuru.com/en/
http://www.globaladvancedcomm.com/
https://www.econnectjapan.com/
https://www.sakuramobile.jp/
http://rental.cdjapan.co.jp/
http://ninjawifi.com/en/

Some suppliers expect you to book ahead. They then prepare the paperwork, return envelope and charge the device ready for your arrival, so it's a plan to do it in advance.

There is variance in devices, speed, capacity and what happens when you go over any limits. Your ISP may have a feature that you can log into that will tell you how much bandwidth you have used in the last week/month of normal surfing, which can help guide you.