If keeping it lightweight and simple is the objective, then my vote goes to the 705.
I have both the 705 and an Explorer SE, and I put a lot of time on both of them. They are very different by the virtue of their respective technologies, but the ability of running very low frequency is what gives both machines the upper hand when hunting copper, silver, or other high conductors. The FBS machines (E-trac/Explorer/Safari) are all able to utilize 2.5kHz, and the Xterra 705/505 can run 3kHz coils. When you consider George Payne's research showed that silver dimes responded best to 2.7kHz, then it's easy to understand why both machines can kill on copper and silver.
Where the FBS machines have an advantage also contrubutes to a noticable difference in weight. Being a multi-freq machine, that delivers as much information to the user as the Explorer and E-trac do, a lot of power is required. But with that power comes fast processors and the ability to see lower conductors like nickels at the same time you're looking for silver and copper. The E-trac is the fastest of the FBS units, and it has programming that enables it to try to "normalize" good targets to the 12 line, which is an especially nice feature for coin shooters
With the Xterra a coil change is necessary for best results on nickels. Fortunately it has that option with the 18.75kHz coils, which also work extremely well on gold.
As far as depth goes, I see little if any difference in ability given roughly equivalent coil size and configuration in my ground(s). I often hit sites with six or more setups between the two machines, but the 705 is my "go to" machine. It's fast, light, has GB Tracking and GB Tracking Offset, has Pinpoint ID and Pinpoint Sizing, has auto Noise Cancel, choice of 1,2,3,4,or 29 ID tones, has Prospecting Mode w/adjustable Iron Mask, and has Beach Mode. It can be turn-on-and-go if you want, or be complicated and more flexible if you choose to tweak it or experiment.
Bottom line is that there are no bad machines on your short list. I'm not a fan of the Safari though, but that's because it doesn't deliver enough more information to the user than the 705 to justify the weight.
Really, you just need to decide what features are important to you and buy the machine that most closely fits the bill.