Go with the lap quilts but make them obviously a set with some theme, fabric or color in common. Mama (a child of the depression and a trained nurse) instilled in us the necessity for planning ahead even if (especially if) facing tough realities is involved. Here's what I see in the case of a 50th anniversary quilt gift.

There is a real possibility that one mate might have to be hospitalized, rehabbed, etc. and the other serves a supporting role. I can see the love when both are using lap quilts in such a scenario that obviously go together. That's just one thought beyond regular home use down the road. It isn't always pleasant to think like this but I have been grateful to Mama MANY times that she trained us to face (and plan ahead for) the possibility of difficult realities.
Yes, I vote for a lap quilt "set" for the lucky couple!

I think the lap quilts are a bit more personal. I'm sure they would love them.

For our friends 50th I made a very special wall hanging that was specific to their marriage and our friendship. Each block represented a special meaning or event. It wasn't large like a quilt, but it holds special pride of place in their home.

If you decide to make the king-size quilt, maybe a double wedding quilt? think about doing what I plan to do:
Make the entire quilt top, then put 1/3 of the batting in the center section (with all of the lining) and quilt it. You would only have to roll 1/3 of the top and lining, so it wouldn't be thick. Then put 1/3 of the other batting on the right side, butted up against the other batting in the center. You could use the batting tape that is for joining batting, or not. Quilt the right side. Then turn your quilt around to the left side, which will be the right side now. Add the batting and quilt this side. Then, you are through quilting and don't have to pay for a long arm. I hope you can understand this. I made it up, so there is no "tute" on it yet!

Myself I would rather have the His and Hers throws!!