re: I want to replace the lead-acid batteries in my lawnmower.
NiMH flat voltage is great for electronics like digital cameras that won't function if voltage drops below a fairly high value, but for things like motors that will keep running to a much lower voltage, the total energy available is more important. NiMH doesn't have a great energy density, considerably less than plain alkaline for example, but the flat voltage makes them work longer in some applications. I'd go with the SLAs, but try not to run them down too far or overcharge them - they don't like that.