People searching on a smartphone don`t have time to look at all your desktop content, if there is too much information.
Mobile marketing use to be the ‘the next big thing’ only the reality is its already here today and with mobile search set to take over from desktop, it has to be taken seriously now for businesses not get to get left behind.
The answer is it depends on the existing traffic coming to your desktop website now, as if its significant (We would need to look at your Google Analytics account) then there is every chance you would get a real benefit in sales and leads from providing a great user experience, which was not the case previously.
This is a great question of which you will get different answers depending on who you talk to.
We believe if your budget is tight then there is no question you should always go for a mobile site (Preferably customised as well as responsive) as assuming you already have a significant amount of website visitors you have a great chance of converting more of them into sales and leads.