The Viggen they are modelling is an attack version. It can only carry sidewinder for self defence. Due to this, I think the MiG-21bis is the better fighter/interceptor. The Viggen we are getting is from the 90's so it's more modern than the MiG-21bis, although the airframe was developed back in the 60's. Performance wise, you should not expect the Viggen to be a great dog-fighter. It's performance (turn rate, rate of climb etc.) is similar to the MiG-21bis as far as I know. Some traits are better, some are worse, but overall they are similar. If you compare the ground attack capabilities the Viggen is superior due to more modern systems, weapons and larger payload capacity.