In addition to the above points make sure to ask the suppliers about warranty. Most will offer 4 to 5 years on a sliding scale. Remember that the cheapest to buy isn't necessarily the cheapest to own. The cheaper batteries will not last as long as the premium brands.
Bolt on cells are easier to install although burn on types will work equally well. Most machines will accommodate several different capacity batteries. The old batteries may be speced right but if your application has changed in anyway, for example you have increased the height of your racking or you have extended the working hours of the machine per week, then considering a higher capacity battery is a worthwhile exercise.