Of the the two, the E-Trac.
But of course, I can't leave this one alone with just that answer.
The XTerra 705 does have a beach mode but being a single frequency detector, it is going to have harder time gorund balancing in the wet salt sand. In dry sand it will be fine. The effectiveness of single frequency detectors can vary from beach to beach, depending on the amount of minerlaization in the sand. Salt in wet sand is a further complicating factor to the extreme. In order to get a reliable ground balance, you often have to dial down the sensitivity quite a bit, which costs you depth. Even then, the detector can be very "chatty".
The E-Trac, being a mutli-frequency detector, will function much better on both the dry (in terms of depth) and the wet salt sand (stability and depth). And it will be fine if you only plan on taking the E-Trac to the beach on a rare occassions. If you are going to be a regular beach hunter, you need to get a waterproof salt beach machine.
The wind on the beach carries fine sand particles that will work its way into the E-Trac over time which will affect the machines performance and could cause it to stop functioning. You know that gritty feeling you have when you leave the beach, even if you didn't go into the water? Well, your machine is no different.
Also, if you drop an E-Trac into the surf by accident, or if you get hit by a wave, kiss your E-Trac goodbye and it will most likely NOT be covered by your warranty.
Hope all that helped.