Which is better to register, this-is-my-domain.com or thisismydomain.com? I am interested to find out which one will tend to rank better in search engines.
If you see from SEO perspective, then neither of the domain names really matters. You content, visitors and backlinks would determine the ranking on search engines.
Domain name is not so important for SEO. thisismydomain.com this is easy to remember and you have to post unique content related to the domain and build good links and keyword targeted.
Generally speaking, Domain Names without hyphens hold a higher value. I would always suggest using the non-hyphenated version. It's shorter, easier to remember and looks better written down. It's not the end of the world if only the hyphenated version is available, but my preference is for without!
If I can I will always take both of them in case competitors try to occupy the other and confuse my customers.
This is also a very good point which I neglected to mention. Get as many as you can afford. Dealing with domain disputes can become a very long and difficult process. Don't take the risk! Register as many domains as you can from the start.
I believe this is your second thread about that , if you insist to use this domain name then is better to go with domain name without hyphen because whenever people search you , they won't rememer to type the hyphen and it look so unprofessional with so many hyphen and sound spammy to searcher and google.