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Do both. Talk to the hr arrange it in such a way that you can give 2 months to both firms. Then wherever your chances to get a job are good take it. In my experience tier 1 tag gives you a lot of opportunities. So don't leave that for sure.
Do you want to be:

Small fish in big pond? Or big fish in small pond? Both has its advantages and disadvantages. But depends - what do you want to be?