What lens to buy, advise neede

[neuroanatomist]

Given that you're using the 18-55mm which is an EF-S lens, I'd recommend the Tamron 17-50mm f/2.8 (non-VC) from the above list. 24mm on an APS-C camera like yours isn't wide angle, and that can often be a problem indoors where space can be tight. 50mm f/2.8 on APS-C will work for portraits (although longer would sometimes be better). The optical quality of the 17-50mm is also very good.

I'd suggest looking over your past shots from the 18-55mm - how many of them are in the 18-24mm range or 18-28mm range? Consider that with a lens starting at 24mm or 28mm, you might not have been able to back up enough to frame those shots.

Another lens that works well for both low light and portraits is a fast prime, such as the 50/1.4 or 85/1.8. Something to consider down the road. Personally, I started with the EF-S 17-55mm f/2.8 IS and the 85mm f/1.8, and that was a great and versatile combination.