I will be traveling between England, Scotland, and Wales May 7-15. Are there pros and cons to purchasing a Brittail pass vs a Eurail global pass?
You need to do your homework to determine if the pass is cost effective. These days most passes are not especially when reservations fees are added in. The British might be since it is the most expensive train system in Europe. But you have to run the numbers.
There are a number of recent posts to show it doesn't pay.
Unless you are a businessman travelling on demand it is not at all true that the UK is the most expensive system. That is no more than a soundbite.
As a tourist you can plan ahead.
An example from the real world- last week on unrestricted tickets I travelled the equivalent of Seattle to Pittsburgh PA for a total of £76.15, and got £5.45 back on a delay repay claim.
That is not expensive. Advance tickets would actually have cost me more!, and led to hours of being sat around waiting for trains.