what you're going to use it for and how far makes a difference as well. Unless you have money to burn and are prepared to clean.maintain extensively then top end would seem out of place on an every day commuter/utility. If you're serious about racing or just wanting something nice for the weekend/Summer then top end becomes more attractive.
105 is good but the 10 speed limits you a bit if you want to alter things, whereas 9 speed intermixes with MTB rear mechs and cassettes. As did the previous incarnations of 105 and Ultegra in 9 speed. For every day use I'd plump for Tiagra, for more rarefied use then 105 or above.