I get asked that question alot. Honestly I can't say one is better than the other. They both have their strengths & weaknesses. It comes down to what type of music you are playing and also how much control you want over your sound. I can say that the GT-10 is highly customizable and it offers way more settings than the X3. To some people that is a turn off because they don't want to get into deep editing of their tone. They just want to plug their guitar in and sound good right off the bat. The sad thing is the stock sounds that come from the factory in these modelers (both the Line6 & Boss) are not very good so most of us like to be able to change the settings to suit our needs. In the end it is you who must decide which is best for you because what is best for me may not be the best for someone else.
Never believe that something is what it is just because someone says so. One must find the truth for themselves. Only then will you know what is the best!!