Acid malt is either infused with lactic acid or is naturally acidified by bacterial action. For the most part, one can assume that the 'active ingredient' is lactic acid in acid malt. But in the case of the naturally acidified malt, there could be a variety of other acids in low quantity. AJ Delange likes to say that acid malt adds nuances that the refined lactic acid from the bottle does not. He may be right. But for most users, the acid malt addition is so small that the opportunity to actually taste those nuances doesn't seem feasible.
Using either acid malt or lactic acid from a bottle should be roughly equivalent. My biggest problem with acid malt is that the amount of acid delivered with the product can vary, whereas the bottled product should be more consistent.
Use what you like!