So lets dig into each one and think about it for a little bit okay. So lets start off with m.domain.com websites. What is a m.domain.com website first. A mobile website is a m.domain.com site. Are there issues with them? Oh hell yeah there are.
You are gonna have redirects that are gonna give your site all kinds of issues. Yeah, you can use the canonical link tag. Google might respect it are you willing to take the chance. The algorithms Google use is known only to a few select engineers at Google. So do you really want to take the chance? Read more about it here.
Next have you ever been on a social media site where your friend has shared something from their phone while you are on your desktop? Yes you have and you open the page and it is a mobile site that is not detecting your on a desktop and thus giving you a crappy experience? That is a fail as well. As your grandmother joins social media she will stop sending the cat pictures and just share the hell out of them. Get ready to be social media spammed more and more by your parent.
The reason behind this post is that I had a potential client that I passed on that wanted a mobile site, a tablet site and a desktop site. I could not get them to just have one responsive site. If you have a m.dot site then you will end up with the potential of taking every page you control and creating that page in a mobile site. You really want to deal with the potential of spamming your own site? The potential of dealing with a ton of crap that you would rather spend your time on other bigger things that can impact the site for good? Make that choice.
Simplify Your Life
Every web resource should always live at one URL. It provides simplicity, search-ability and user-friendly behavior that has enabled the web grow into what it is today.
User Detect and Render
This is where the user comes to the site and you detect what device they are using and then you render it using. Mozilla has actually come out against it. Read more about it here.
So where do you think my mind is at on this subject. Bingo, bingo I like responsive websites. I could care less about m.dot or m.domain websites. I want to see them dead. Who wants to deal with the issues of subdomains and duplicated content? Not me. So moving on I like a site that is rendered on desktop, tablet and phone. So how do you build for all of them? Build for the phone then tablet and then desktop. Keeping in mind that users expect that a phone is supposed to load as fast as a desktop at home. So keep it super simple, do not go crazy.
Responsive sites add a luxury of one single URL, so simple no major issues that are gonna keep you up till 3am trying to figure out how to get the mobile site out of the index that are showing up. make your choice but mine is responsive and I hope yours is as well.