Dan, I do have the MB Spiral Down. I can't tell you much more than what's in the post Ashley linked to because I haven't used it in the field. I still have it (60 day return policy) but in the interim, I got a Summerlite on ebay for a really good price(seems like a few summerlite popping up there lately – keep your eyes open) that I will probably use instead.
I really like the Spiral down at home, but it is that 2/3 zipper that gets me. I like the textured fabric more than the slicker Summerlite fabric, but that's not to say the summerlite fabric is bad. The MB definitely has more room to move around in, but you sound like a skinny guy, and as a side-sleeper myself, I don't find the summerlite too confining. I'm 5'10" 175, with fairly broad shoulders. In either bag, there was enough room for me to wear a MB down inner park or even Alpine light jacket + heavy fleece pants to boost warmth w/o being squished.
If the 2/3 zipper isn't a deal-breaker (I get hot feet and need venting options) than for the price the MB bag would probably be great, unless you find a summerlite on ebay at a good deal.
If you want pictures of the bags side by side or anything, let me know. I should have time on Sunday night.