I'm an advocate of the 21/70 combo for travel. However, I also have the 15 and, until a couple of weeks ago, the 35 with me. I've sold the 35 in favor of the 43 as I didn't have a fast lens in my bag, so a 4-prime setup. Honestly, they're so small it's hard not to take all four! I don't necessarily like having four choices, and if I fall in love with the 43, it might be the natural partner to the 21 instead of the 70. Then, the 15/70 would be in the bag for particular situations.
I really think the 21 is near the perfect length for street/travel photography (I'm not a "classic" street photog, I only do it when I travel). And the 70 is great for details/candids/the odd portrait. I carry the 15 for the times I want a particular shot, but I don't walk around with it on the camera.
Basically, I've taken another look at my travel kit just as you are, and I've come to the conclusion that this is what I'm trying to put together:
Fujifilm X100 (23mm f/2...drool)
K-7 with DA 15/43/70 and the 50-200 WR
All of this will fit easily in my Timbuk2 messenger with the padded insert from my Kata 467i, and give me:
Compactness, light weight (relatively...I love when I Google "travel lens kit" and get forum responses from C+N users hauling three huge zooms, like the 14-24, 24-70 and 70-200...holy crap), image quality, fast lenses (2), pretty wide (I'll leave my 12-24 at home for landscapes), length if I really need it, WR if I really need it, and a backup camera...I envision city walking with the X100 in my hand and the K-7 in the messenger bag with one of the LTDs on it, chosen according to my mood
Anyway...huge tangent...sorry
But if I wanted to get to two lenses and one body, I'd most likely go 21/70.