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.
You know what, I bought a Yuasa from the local Yam dealer and it packed up after a week, so while I was waiting for a replacement, I bought a 30 quid job from some car parts dealer. Well, guess what, 2 years later the damn cheapo battery is still 100% fit and the 90 quid Yuasa (dealer prices eh?) is sitting in the kitchen cupboard waiting to be fitted. Who knows, the cheap one might drop dead tomorrow, but it's done two winters and I could get 3 of them for the price of the Yuasa. It's good to have a spare ready though. I've also got a Motobatt on the other bike and that's been rock steady for 4 years.
And when I say it's done two winters, it's sat at the train station in sub zero temperatures and only been ridden for 6 miles per day, hardly enough to keep it charged for the next start.
I dare say it'll let me down at the most inconvenient moment, but if you only intend to keep a bike for a couple of years.. what the heck.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....