I'd go for the Ultegra equipped version but it's £300 more than the 105, and personally, the 105 is pretty good as it is, especially when it's £115 more than a Tiagra equipped one (whom I really disliked the brake shifter).
mix and match is always good providing it work well with each other, e.g. 10spd 105 derailleur with 8 speed cassette.
I'd go for the Ultegra equipped version but it's £300 more than the 105, and personally, the 105 is pretty good as it is, especially when it's £115 more than a Tiagra equipped one (whom I really disliked the brake shifter).
mix and match is always good providing it work well with each other, e.g. 10spd 105 derailleur with 8 speed cassette.