Well, though I have not used either of them ( I am in fact, a Videocon D2H user), yet based on what I have seen at demo and as a personal experience, I would suggest you to opt for Airtel Digital TV. Here are my reasons for the same -
1. Reliance seems to have major issues with its set top box. The interface is believed to be slow. It has been observed to heat up faster. Airtel has better hardware compared to the competitor.
2. In terms of picture quality, reliance has a slight edge over Airtel . However, the difference is minute between the two so that they can be considered equivalent.
3. Customer care and after sales service of Airtel is better in comparison to Reliance. Customer care executives lack professional behaviour.
4. Reliance offers comparatively less number of channels. There seems to be a space crunch being faced by Reliance.
However, Reliance has the distinction being more effective against rain fade. While other DTHs suffer severe black outs during rain , Reliance digital TV resumes the services within seconds.
Airtel has dubious distinction of having costly packages. It could be one of the factors that may move a prospective customer away.