I have owned a 3880 for about 18 months now and it has never had a clog. I'm on the third set of most inks (and 4th set for a couple of them). My printing tends to be sporadic and I've left the printer off for as long as 3 months, started it up and printed (though if it has been that long, I usually gently agitate the print cartridges though given their design I am not sure that this is really required). My printer is in the family room which is the lowest level in our house here in suburban Washington DC. I do not do anything special to control the temperature and humidity other than running a dehumidifier in the summer to keep it down. Winter, spring, summer or fall, nothing seems to bother it at all!
As to the difference between the 4900 and the 3880, there is clearly a design difference on two counts: 1) two extra inks and 2) a different print head that has smaller droplets of ink. It may be the head design has something to do with clogging. If you are going to be an intermittent printer, you should go with the 3880 (it's also a lot lighter than the 4900).