Blockley for me.
Salisbury plain can be a bit boring?
You are right there - SP is a 'Plain' so its mostly tank tracks ect that the ride will go on.
I like it as thats where I did a lot of my riding BITD but it is a long way for someone to travel//pay ride for something not that exciting - although the 'event' is well organised and gets you on parts of the Plain that are normally out of bounds.
The SPAM biking events at Erlestoke though are a different matter - the SW region guys to those - well recommended.
I did the first ever one of these called the 'Salisbury Plain Challenge' BITD - must have been around 88/89... they were the days!
I vote do Blockley on Saturday and those that want to can to Sarcen on Sunday.
The Blockley ride is great fun, with a nice pub halfway round