My opinion you’re given first one is better then 2nd option, main keyword.com. It’s always helpfully to rank high if the main keywords is included with it.
You could select anyone based on your need. If you think both the words are meaningful, you can go for any one, as both are good for SEO. People normally say that domains with "-" will not work for SEO, but its false.
mainkeyword.com is much stronger. While they might be close for SEO purposes, the one without the dash is much easier for people to remember (and link properly to).
Both are considered by Google to be exact match domains for the search phrase "main keyword". So as far as rankings, they will both be equally effective from an SEO perspective. But non-hypenated domains are shorter, easier to type, easier to say out loud, easier to remember, and look MUCH more legitmate. So from a user perspective, the non-hyphenated version is best IMO.
Ok mainkeyword.com is the best choise! but if .com not available - .net or .org? and what next, if those not available? I have idea making video blog, but for my keyword com,net and org not available. so how about .tv ? It's strong for serps?
im sure i read something somewhere that said google confirmed it treats main-keyword.com the same as mainkeyword.com