I
Don't have budget constraints. Important things are
1. Addition of new hd channels
2. Picture quality
3. Service
Tata sky has largest librerary of hd chs and they are fast in addition of new hd chs too. More chs = more cost so Tatasky is costlier than Airtel .
Upto 44 inch I don't think you will see any difference between these two .
Service will vary depending upon location. In some places Airtel guy will reach your home earlier than Tata but in other locations Tata will reach before Airtel.
One thing about Tata sky is that they provide special hd feed during important events like star sports add-on , or during film festival or multi camera view during live matchs absolutely free to customers.
I am not sure Airtel provide that or not. Airtel users can tell better