The 968 is a highly rewarding car, extremely competent, built as if hewn from solid granite and great fun to drive. It will also be quicker than your boxster!!!
comparative maintenence costs will in part depend on where youve had the boxster serviced. you'd be barking mad to get a 968 serviced at an OPC - parts and labour are ruinous and since OPCs dont sell 968s any more, the pool of experience diminishes year by year. If youve had the boxster sorted via OPCs, then the running costs of a 968 through an independent will be less. If youve had the boxster serviced through an independent already then be prepared to budget for a little more.
Having said that, if you seek out an example with an impeccable service record you should be ok. Belts need doing every 40-50K, budget £400+ for this. If the cam teeth break (rare despite the many apocryphal stories on the matter) then youre looking at £1-1.5K from an independent, £3K++ from an OPC. Apart from cams however they are pretty reasonable. clutch is extremely easy to replace (unlike the 944) and I cant think of any other obvious achilles heels. electrics, sticky windows, starter motors, all niggly things but hardly budget breaking.
Dont know how much your boxster is worth - ?£20K. You are probably going to free up £5-7K.
Opinions vary on the best model. My cab is very user friendly and the standard lux trim essentially gives you a modern car with little hassle and huge driveability. If youre feeling bonkers then the clubsport is the one, but the ride is very much on the firm side and it may get a bit wearing, especially after a hard day at the office. the sport has the same suspension as the CS so a similar "firm" ride but then of course you get HANDLING in return. If its a daily drive youre after I'd go for standard lux trim or the cab which Ive found superb. Others I'm sure will disagree
Money issues aside, the 968 offers a different though very comparable driving experience to the boxster. Against the 2.5 or the 2.7 I'd lay money on the 968 any day, irrespective of financial considerations. The 3.2? well, thats a different story. its also a whole lot more money. Summarising, I defy anyone to beat the 968 driving experience for similar money.