First of all, you've got a very good offer on TCS scale.
Now, what Centre of Excellence it is? Oracle Apps? Java? .NET? What?
TCS has COEs in nearly all its centres nowadays
Believe me, the term COE is acutally simply "nothing".
At your experience level, if you are specially joining in COE purpose, you'll have following responsibilities (may differ depending on which COE).
* Writing proposals for new projects on your COE field
* Giving/Organizing training to new recruits on this field.
* Undertake project management jobs
Usually, at your experience level, people don't do "coding" in TCS. Depending on requirement, you may have onsite chances BUT, if you are offerred a very high position in COE role, you'll find it difficult to get release from the project.
Onsite is often a matter of luck (and some politics in higher level), so quite difficult to predict on that.
However, Pune is the TCS's best R&D centre - TCS's most PhD holders actually work in Pune.
If you are happy with salary and not dying for onsite immediatly, then I suggest you take this offer.