I did exactly this earlier this year, one each way - SEA-AMS-LHR on the outbound, LHR-DTW-SEA on the return. I liked splitting it up that way, but if you were buying a J business class fare, it's no contest - you'd absolutely want to take the DTW routing both ways, because the 767-400s Delta flies to LHR have true lie flat BusinessElite seating.

In coach, though, it's more complex. One consideration is whether you have lounge access (and as SkyTeam Elite Plus, you should.) If so, AMS starts to look a little more attractive - you end up leaving SEA midday and arriving AMS early morning in Europe. If you time it just right, you can probably jump in the KLM club in AMS and take a shower before your connection. The SkyTeam lounge in LHR is ridiculously, absurdly nice, but I'm not sure if you can access it on arrivals if LHR is your final destination... and a shower in DTW isn't going to do you much good since your journey is only half over.

The other gotcha to watch out for is your connect time in Detroit on the return. I ended up stuck there for some ridiculously long period waiting for the 7pm flight home to Seattle - again, great chance to take a shower, but it really started to drag on for the last few hours of the layover.

Finally, the KLM flight from AMS-LHR was, how do I put this - lame. It's a really short flight, so hard to complain, but certainly don't prefer AMS just because you get to fly KLM when you could be sitting in Delta's far superior domestic F for part of your journey SEA-DTW.

So in short, my advice for a coach passenger from SEA would be to optimize for convenient timing and lounge access/showers. For me, that was AMS on the outbound and DTW on the return (in other words, "primary transatlantic leg first, connection second").

Hope those ramblings helped some...