I have 105 on my winter bike and Ultegra Di2 on my best bike. Can’t fault either, to be honest.
Ultegra is a bit lighter and looks a bit nicer than 105, and maybe the brakes are a bit better, but the shifting on both is outstanding.
As for 50/34 versus 52/36, you’ll surely have a good idea on what you need based on what you have now? My 105 is 50/34 and 11-32. My Ultegra is 52/36 and 11-28. With the semi-compact Ultegra, I sometimes find that I’m between chainrings on the flat, particularly if it’s really windy, but I actually prefer climbing on it than the full compact 105.
YMMV, of course. Depends on where you normally ride, your weight, riding style, etc. I guess if you’re unsure, go for 50/34 – better to be spinning out on fast descents than running out of gears up steep climbs…
EDIT: Seems I was a bit too late! 😀
50/34 and 11-28 sounds perfect. You going to be able to get tyres in time for a bank holiday shakedown?