Brief 4 - Upper/Lowercase?

Trying to decide whether to use just uppercase letterforms, or just lowercase, or to have both. Decided to look at the complete alphabet in each typeface using both upper and lowercase to help me make my decision.

Helvetica Neue Heavy

Futura Condensed ExtraBold

Gotham Ultra

Univers Extra Bold

Gill Sans Ultra Bold

I definitely don't think I should use just lowercase letterforms. I think using just the uppercase would provide sort of a more solid area to doodle in as they will each individually be the same height and what not as each other. Hmm. I will use just uppercase, but if when I come to experimenting with layouts for the doodle books it would work better with both, then I will consider using both, but for now my decision has been made.