I think that it really depends on your business which I have limited information. In general, my thought is follows:
(1) Single Form used in many Landing Pages
If the information needed for most of your landing pages are pretty much the same, then this is a good option. If you decided to collect additional information say a lead's interest 6 months down the road, all you need to do is change that one form, re-approve your landing pages and you are done.
This also works great if you only have a small handful of forms
(2) Form for each gated asset
This works better if you have very different information you need to collect for each landing page. A good example would be that you have multiple lines of products. For example, if I own an electronic store (which I don't), I might ask the lead what TV size screen they are interested in if I am doing a special on TV while I will ask them how much storage they need if they are buying an iPad. Not sure if this is a good example but you get the idea.
In this case, there are just too many variants to use one form and this option is probably better.
Hope this helps.