bought the others back on yussa,
I had a motobatt, didn't last 6 months, so back on the Yamaha recommended yuasa ,
I wouldn't pay the extra for a lithium battery but nor would I buy a budget battery for £30 as you tend to get what you pay for. I've usually gone for Yuasa or Varta and they have never let me down lasting at least 5 years with minimal maintenance required. Following good feedback I bought a Motobatt for my Gen1 which has been trouble free for the past couple of years.
Also, in Performance Bikes a few issues ago, they fitted a lithium battery to a big twin (Ducati I think) and they said that the lithium battery spins the engine up much quicker, a bigger initial thump as it were, which places less stress on the sprag clutch. Or summick. Anyway, that could be one good reason for going with a lithium battery, but I've seen the prices and thought.. er... no....
Yuassa's can be found online for £45 and Varta's for £40, I wouldn't pay £90 for one but I know sometimes desperate times call for desperate measures. You could get lucky with a cheaper battery but I personally wouldn't take the chance.