Honestly... Pick whichever one you like the look of the most, they're all good cars, and the two newer ones are proving themselves already... Set ups will develope as time goes on...
I personally think the C4.1 looks the best, i loved the S2 back in the day though so thats why
Alternatively... Keep your B4 as is and try an ansmann x2c for the price of the other conversions...