TN and IPS are remarkably similar. It's very common for the differences between the two technologies to be blown completely out of proportion given that IPS is a more expensive and superior technology, but most "ordinary" people simply wouldn't be able to tell the difference between them.
In a nutshell: IPS has superior viewing angles and better colour vibrancy
TN has better response times and can have better black quality (no "IPS black glow")
The good thing about TN is that it's much cheaper when you go for larger panels, which is what I would suggest be the determining factor in making a decision. If you want a bigger panel like 27" or 28" go with TN; if you're happy with 24", then the Dell U2412M would be the obvious choice.