Ditra and Hardiebacker are underlayments. Suntouch is a heating system. You need one of each.

I'd pick Ditra over Hardie every day of the week.

I'm assuming that your contractor is referring to Schluter Ditra Heat? If he/she is then that's a great product. The underlayment/uncoupling is part of the system so you don't need anything else.

Here's a link to their page...call their number too if you have questions. Super helpful anytime I've called.

http://www.schluter.com/ditra-heat.aspx

For Ditra be sure to use a megohmeter to test insulation or you won't have a warranty. Chances are your electrician will need to dig through his/her garage to find theirs. If your installer does not have the meter say good bye to them. The uncoupling properties are great but you may prefer to use a different heat cable manufacturer. Others will offer a full warranty and fit into the Ditra Heat mat.