[meeera]

I am guessing that you are in the USA, since you mention looking at Nooks in the bookstore. It is useful to put your country into your profile, as this is a global forum and answers may vary with geography.

If the SD slot is a flexible preference, and you're not too fussed about fonts and fine-tuning display preferences, you might find the Kindle Voyage a good choice for your needs otherwise. It has the choice of touching the screen or using the haptic buttons. It is also very expensive. The Paperwhite is a cheaper option; no buttons, just touch.

If you want the very smallest/lightest mainstream reader that still has a full six inch screen, the Kobo Aura might make it to your list. It doesn't have the best screen compared to current readers but it is perfectly readable. Secondhand it would be a budget option.

If you're after a larger screen and don't mind a little more weight the Kobo H2O is lovely, and is also coffee-resistant if you juggle a cup on your commute. It has the ability to highlight and annotate as well as to import these annotations into Calibre, but highlighting is a little quicker/smoother on the Kindle.

Kobos also allow you to manage your collections with Calibre, if that's useful to you.

Comfort of holding depends on device weight and on the case you choose more than it depends on the shape of the reader itself, IMO.