Not quite, but the mistake isn't what you think. When comparing things you must use "than" not "then". Otherwise, both of the sentences are possible.
Both sentences are of course hypothetical, but:
Sentence 1. uses -ing which implies that speaker things that the stealing would have happened over a period of time perhaps there would have been several acts of stealing.
Sentence 2 does not use -ing. This does not, however necessarily mean that the theft would have occurred all in one go although it certainly could happen like that, but that the time over which the theft would have occurred is of no concern to the speaker.
To all intents and purposes there isn't a great deal of difference between the two sentences.