I second what
@Cyclist33 says - forget the marketing, it's all about the set-up.
I ride Tiagra 4600 and it goes like a sewing machine; smooth, quiet and reliable. I've done about 3,000 miles on it and have never had a chain come off or even a missed shift in all that time, except for when my chain wore out recently and it started skipping gears - but that was my fault for letting the chain get too worn, and now with new chain on it's back to being flawless.
By contrast I've sampled 105 on my friend's Cannondale and it clanked and creaked all over the place, despite being well-oiled and (allegedly) recently indexed. I'm sure a good look at his mech could have had it tuned up and running as smoothly as mine, but I don't see how it could be made to run any more smoothly or reliably than mine.
The Tiagra (unless it's the very latest, which I haven't seen) is uglier of cable and hood, so you might decide it's worth the extra £100+ for the aesthetics and/or 11 speed option.
When I'm in a position to get another new bike I'll be looking to plough the funds into the best possible frame and wheelset; groupset will be a comparatively minor consideration.