Better than both of them is an emerging technology -- the Ultracapacitor.
http://www.ultracapacitors.org/ultra...tor-works.html
Becasue UCs are electrical devices, they offer the potential to work with the battery system by acting as a surge buffer. This benefits the battery system by providing slower changes and lower currents (read lower temps) and also benefits the drive train side by offering short, large bursts of energy for acceleration and absorbing same for deceleration and braking.
IMHO, the next major step in hybrids will be a(n) UC in series between the electrical interface to the drive system and the battery bank.