Oh boy that's a tough one. I've never grown either of those..I'm new to dahlias as of last year but I can understand where you're coming from. I spent a good two weeks narrowing down my selections. I felt like I was studying for an exam. It was exhausting!

My vote is for Day Dreamer, I like the rounder petals, looks more romantic :) and I'll have to add that to my wish list for next year, so pretty! But I went with mostly pinks, purples and whites for this year.

I have not grown either of these, and not likely to, as they don't have much show
potential and I grow similar ones that suit me just fine. However, it
is a safe bet that both are good tuber makers and tend to have a large number of
blooms at one time. I offer this educated guess because Gitts introduced both of them, and his family business still sells them over a decade later. SI does not bother with varieties year after year that don't have those two qualities, as they only offer around 200 varieties a year, and have to drop several off their list each year to make room for new ones.

There are many waterlilies that resemble those two, and they all have their own little idiosyncrasies that either pass without much notice, endear themselves to the grower, or peeve the grower. Some, in that similar color range, are Monet Glory, Cameo (two of the best for show), Chimacum Cathleen, Bracken Lorelei, Figurine, and the list can go on.

Junco, it sounds like you might just have to take a dive and try both
side-by-side. ;-) I do that often, and find that I 'can't do without'
either (sheepish grin). Please report back in the fall, if you do decide
to grow both, and tell us what you think.

Day Dreamer is one of my most favorite dahlias. It is VERY prolific, I can't even tell you how many flowers I got from it last year. I only keep three plants of it since I get enough flowers from those to keep me happy. Good tuber producer as well. The plant itself is probably one of the bushiest I have - lots of laterals for all those blooms! However, it really needed staking because it got SO big and SO heavy. The flowers come out a pale lemon yellow, as they age they become more peach. Lovely colors. I have added photos: 1. Mixed bouquet with Day Dreamer, Nicholas and Touche dahlias, 2. A Day Dreamer bouquet, 3. Day Dreamer with Gerrie Hoek.

I haven't grown Summer's End. I saw no reason to get both as they seem to be very similar. The big difference seems to be that Day Dreamer has rounder petals, and Summer's End has more pointed, and perhaps Day Dreamer has smaller flowers. I have a lot of cactus dahlias so I like the roundness of Day Dreamer as contrast.

Oh, this makes me long for summer......