After reading through some of the stats the first source comes from Psychology of Women Quarterly and they promoted/infered the idea that the woman should be older(which is biologically an issue) so I'm not exactly sure of the quality of the research, plus it's important to factor in who's funding the research.
Now the second study posted by MarketWatch I would probably give more credence/credit to(not the TITLE MarketWatch decided to use), as they stated that age gap was not the only factor nor the deciding factor, but when/if the couple has children, and if they are able to last 10 years(94% more likely to stay together for life if they last 10 years). There were also other variables like children out of wedlock.
The pure stats just show that you can't apply rigid rules either way(significant age gap or no significant age gap)