Smaller coils are good for trashy areas or a lot of "hot" rocks, but they give up depth. A larger coil will find deeper targets, but make it a little harder to pick out the good targets in trashy areas. Have never used Fisher products, I have a White's MXT for coin,jewelry,relic, & prospecting work, and just picked up an MX-5 as a back up, and depending on performance, may relegate the MXT to back up status, as we don't have much nugget shooting in my area. These are both in the 14khz range, so I suspect the 19khz will be a little better on gold. The higher the frequency the better for detecting gold. For a general use unit, for me, the MXT in any of it's versions is hard to pass up. Has been said that once you get used to it and know it, you won't be "out gunned", especiallly for general detecting. There are other specific gold machines on the market that will out perform it, or others in that general vein. The best advice is to find a dealer that handles several brands, have a long conversation of expectations, and try/handle a few before purchasing.
One of the old truisms on detectors is that every $100 is worth around 1" in depth, depending on ground, coils, user experience/confidence with that particular machine