I would say the MXT may have a bit of an advantage hunting for natural gold and small gold jewelry since it's was basically designed for that, but the 705 will be much more efficient on the beach, assuming you want to hunt the wet sand. And it will do very well with a HF coil hunting for gold.
The MXT is not good in the wet sand ( based on my experiences using it....falsed constantly and the only way to possibly run it was to have very low sensitivity and higher discrimination settings which defeats the purpose), compared to the 705 running a LF coil.
That is the beauty of the 705....you can put the HF coil on to hunt gold and lower conductive targets and the dry sand for jewelry, and if you find there are lots of targets in the wet sand, put on the LF coil.
I used the LF 9 inch concentric on the beach yesterday (NJ beaches) in the wet sand. Running at sensitivity 22, it ran very smooth. Found lots of clad...no silver or gold this hunt but thats ok. Remember its not the best wet salt sand hunter, and I have units like a CZ, E-Trac and Infinium for that application, but I wanted to try out the 705 in that environment and it worked very well in my opinion.
No falsing at all in the semi-wet, and very little in the wet and suds. Now mind you, the LF coil is not as sensitive to smaller, lower conductive targets as the HF coil is, but for bigger gold, clad and silver, the LF coil is the one to use in the wet, SALT sand and suds. It runs much smoother than a HF coil running at the same sensitivity and discrimination levels.
I ran in all metal mode for the most part yesterday and pattern 1 sometimes. I like all metal mode on the 705 in the wet sand. Tried the Prospecting mode as well, but it was too noisy in the wet sand so I stuck to Coin mode, all metal.
The 705 has more options than the MXT in terms of ground balancing, tones, settings, etc. With the coil options, it's a better all round unit in my opinion. Based on your post, it looks like an all around unit is what you need.