I think you have been waiting for the new picnic for long time.
I haven't seen the new picnic, but I think it may not have very big difference with the existing one.
To compare Picnic and Previa is not sensible as they are not in the same class.
For Toyota, APLHARD > PREVIA > PICNIC.
I think PREVIA suits you better, the only thing that worries you is parking. Don't worry, you will get used to that within a month of daily practice.
If you are scared with the engine problem of PREVIA,
wait until 2007, the problem must have been solved and you can compare PREVIA with the new PICNIC then.