Is a 30 DD (E) Really the Same As A 32 C?

Yesterday I finally got my act together and measured my breasts using the advice on this sub. After I got back off the floor I talked to both of my partners and made them do the measurements because, well, I couldn't believe it. I always really believed I was a 32 A - and then when I gained a little weight I went up to a 34 A. Yeah. Anyway.

My measurements are: Bust: 35 Under bust: 30.5

It seems I need either a 30 DD or a 32 C. Today I tried on some 30 Ds at Nordstroms today, they didn't offer me any 30 DDs. I also tried some 32 Cs. Anyway, the cups felt good, though I'd like try some of the 30 DDs as well, but the band felt really tight. I am unclear if that is just an adjustment thing or if I really do need to go up a band.

So, how do I know? When you're in between band sizes which way should I err? My breasts are small (despite the DD label), they don't necessarily need a lot of support... but I've been wearing bad bras for so long now I have no idea how it should even feel in order to make the right choice.

Help!

Edit: I've now learned that both bras I tried on were too small, so that makes sense. However, I'd just like to say that even with them being wrong, they still felt 100% better than the 34 A I have been in. I'll be absolutely fascinated to discover what the actual correct size feels like.