I'd go 35L due to that. Make no mistake, it is a hard choice, since for your situation, the versatility you'd get by upgrading from a 24-70L to a 24-105L as a walkaround is higher than what you'd get from going 35/2 to 35/1.4, since you say most of the time you need low-light, you're permitted to use flash. But since you naturally go for 35mm quite often, I'd suggest the 35L.
Good points, Collin. Although, I wouldn't say I'll be upgrading from the 24-70 to 24-105