I just buy whatever I feel is a good price compared to what it’s normally available for generally. However I will think twice based on the reputation (and maybe past experience) with some companies, plus check if there’s a catch (high shipping charges or very slow shipping, OEM version, older model, bits missing, possibly not even in stock despite claims otherwise which I’ve been caught out on a number of times). Likewise if it’s cheap because it’s really a front for an ebay store and it’s coming from China, or it’s a European store then I’ll do a lot of checking first.
If there’s not much difference, I may go for the store I trust more, or maybe LBS if they’re doing a reasonable price, and I want it now.