Not all cotton t-shirts are created equal. I have several thin, light organic cotton shirts from Wal-Mart that I really like to wear. They are more comfortable than the Beefy-T's that comprise the bulk of my t-shirt collection.

That being said, I prefer synthetic shirts like Under Armour (and yes, I prefer that brand to the 'knock-off' brands, but those aren't bad either). If it is on the hotter side I will wear a sleeveless tank if I need my shoulders covered. I actually prefer workout or yoga tops, but they don't play well with pack straps. I have a nice synthetic tank by Columbia that I will wear as needed. I don't have anything sleeveless by Mountain Hardwear, but I love the long sleeve backpacking shirt I have made by them. You might also look for smartwool or similar. I don't know that you will find much weight difference though, if she carried an equal number of shirts. The biggest advantage of synthetic is the ease at which they can be rinsed out, dried and worn again.

HTH,
MNS