IET Lucknow is better. Being a Govt. institute it charges a lower course fee. If you are planning on preparing for competitive examinations like CAT, GATE, GRE, etc. IET will be the right choice. The college schedule is flexible and leaves a lot of room for exploring other interests. Lucknow has good coaching institutes for competitive examinations. At IET, you will meet seniors who have qualified GATE, CAT, GRE, etc. They will be able to guide you regarding preparations.
Both the colleges VIT Chennai and IET Lucknow are good. I can give you some information about the institutes as I have some friends from both campuses.
VIT:
If you think that you can do better and you have no financial problem go for VIT or else IET is not a bad option.
IET:
I hope this helps you in deciding between the two.