Content amount. Simply put, the more cleaning solution you have, the more cleaning you can get done. Getting the biggest bottle of cleaning solution you can afford would, therefore, seem like the best thing to do. Bigger bottles will cost you more initially, of course, but they do provide better overall value. Going for bigger bottles of cleaning solution will save you more money in the long run than if you opted for smaller bottles.
Strength. A stronger cleaning solution is generally preferable to a less-potent product. Strong soap scum removers can usually be diluted, so you could still use them on surfaces that can be worn or damaged by a strong solution. With a weaker product, there really isn’t anything you could do to make it more effective than it is already.
Odor. Soap scum removers are usually pretty strong and abrasive. Most of them will also have a strong smell, due to the harsh ingredients they contain. Depending on the particular product and your sensitivity to smells, you might find some to be a bit off-putting, or even overpowering. If possible, try to smell the product in person before making your choice. If this isn’t possible, you might have to settle for basing your decision on online user reviews.
Effectiveness. A soap scum remover is only as useful as its effectiveness. A good product should enable you to clean soap scum, and other types of grime, without putting in too much effort. Consider it a bonus if it is able to clean other types of build ups as well.
Versatility. Soap scum removers should be able to effectively clear up soap stains and deposits, full stop. However, the best products are able to handle other related tasks as well. It isn’t unreasonable to expect a soap scum remover to be able to clean dirt, grime, and other nasty matter that tend to accumulate on bathroom and kitchen surfaces. If a particular product can effectively clean up more than just soap scum, it is probably worth purchasing.