I'm personally a huge fan of Vtech or Mercedes Swap Shop. Vtech Georgia (the country not the state) has been known in the Mercedes community for years. Price is better than MSS and free re-tunes. MSS however also provides high end quality tunes, great customer service and in the US.

I like them both A LOT. I have tried almost any reputable tuner out there. Best would be to get a Dyno tune but if not possible, Vtech Georgia and MSS provide the best in the business for both ECU and TCU. TCU I'd lean MSS just for the fact that you get 4 modes (Comfort, Sport, true Manual and Agility) - nice feature to have and it can be customized to your liking. Good luck!