I've owned a Spyder, a Spoiler-Exploiter, and will in the future (hopefully soon!) own a custom Series II replica of John Entwistle's Spyder/Exploiter currently under construction. Oliver's done a great job (as always) of laying out the differences, but I will say I've noticed variations in the body styles. They're all of the same angular, pointy design, but cutaways, lower horn lengths, etc. can vary some.
The Spyders are wider at the waist, if you will, than John's cone-head Spyders. Also, the Spyders have individual tailpieces for the strings; the Exploiters have bird tailpieces. The electronics package differences have already been covered.
Playability wise, the Spyder is a big instrument, and not for the faint of back. The case is the size of a keyboard case in height and length, and you really have to stretch your arm to reach 1st position. The Spoiler Exploiters, with a 32 scale, are a bit easier to get around on. The V-head Spyder headstocks are huge - damn near a foot long. You really have to plan your movements ahead of time.
I've sold my Spyder to Dean J. of South Carolina and am eagerly awaiting my Series II. When I get it I'll post a report.
Alan