Hi Iris, I like both but (in British English) they don't mean the same thing, and I suspect you don't really mean the first one. The preposition is all important here, as follows: “Substitute…for…”—first replaces second. “Substitute…with…”—second replaces first. “Replace…with…”—second replaces first. “Replace…by…”—second replaces first. etc. In addition, as I'm sure you know, 'a lot of' is fairly informal, so you could substitute 'many' for 'a lot of'. Hope this helps Rachel

The first sentence is better.I would also replace the words "a lot of "for "Many".