Hmmm been there... done that... good luck with it all...
For a model its the better deal potentially... paid shoots guarantee a return for their time... a % deal means if the site does well then they will earn far far more than just their flat rate...
Example I know the girl from KatesPlayground.com she was on a 50/50 deal making around $40,000 a month... even after takign off expenses and the affiliatte payouts I bet she personally made around $10k a month !
It does mean the girl needs to interact with the site, and sorry girls but most of you models are way TOO lazy ! The site usually takes 3-6 months of hard work to get the money coming in... very few fly from day 1 !
The interaction I would imagine will include the shoots... doing some webcam shows... promoting the site on message boards... doing blogs and the like... not hard work, just repetative, few hours a day to really get it out there...
So you have to weigh up the guaranteed money against the potential of a % of the sites profits...
I have two final pieces of wisdom, for the webmaster put in two clauses, one being that you get to shoot a years worth of content, to include launch and updates for the first year to guard against the model flaking (trust me it happens) and secondly a clause that the site will never have to come down... again some models change their mind and think they can get the site taken down, arguably if its a 50/50 partnership they can... so build that into the contract...
Secondly for the model... make sure the contract assures you the payments you are due, too many times webmasters have set this up then run off with the $$$... not suggesting "disturbed" would... you want access to the back end billing admin so you can check the income from the site, you also want to make sure that any additional revenue streams coming in from the site also get you the % you agree... no hidden sales that you aint getting credit for, and finally make sure that if this isnt a % on the gross amount, and it's infact on the Net, then you get a say in what expenses come out... ie site makes $40,000 but webmaster says he spent $35,000 on a new car that he needed... extreme but you get my point