I forgot about tufo's. Besides 50g or so, what's the difference between the primus and flexus primus?
Rubber on the Pro level tires is both grippier and seems to last longer, Flexus series have a nicer, thinner, somewhat more supple casing. I have raced the Cubus in both Pro series and Flexus series, and would not pay the extra for the latter.
BTW, I think the Primus is a bit of a waste. It's billed as an "all around" tire, but it's crap in mud because of nob spacing (packs up easily), and it's not really any faster than a Cubus in dry conditions. I'd use the Cubus as an all-conditions tire (in fact, I do).
If you want to spend more money, Clement's new tires are awesome. Super durable (made by Tufo), but really sweet tread patterns. PDX is the best mud tire I've ever ridden, and not so bad in the dry. LAS is a really great file-tread, with substantial side nobs for cornering. They all come in a true 33mm casing as well, so they are UCI legal, but noticeably bigger than Tufo 32's.