The NiMH Battery on my Giant Twist is still going strong after nearly 2.5 years and regular use. Those bikes are sometimes available secondhand for as low as £300 ~ £400 if you know the right places to look and as long as you have someone knowledgeable check it out, I know which purchase would entail lower risk, less hassle and best reliability, even with a 12 month warranty on a 'budget priced' new bike!
I bought my first Powabyke in 2000 I traded it in later for a Shopper. I am on a limited budget, at my age there are holidays & presents for grandchildren to pay for. If you need to commute to work it may benefit you to spend a bit extra. Eight years on lead acid battery's is quite a good advert for them so I will stay using them.