The first question I would ask is where and what are you primarily hunting for ? I had a 705 with 5 coils. Loved it. Only reason I sold it was beacuse I am primarily a salt water beach hunter, and it's now beach hunting season and the 705 would not be put to good use. The 705 does work ok in the wet salt sand but I had to dial down the sensitivity and find the right coil combination. Much easier to just use an FBS unit in that environment.
So if you are NOT salt water beach hunting, or if your soil conditions are not brutal, stick with the 705.
If you have to have an FBS unit, check out the classifieds because there are some good deals on used Safaris and Explorers too. The E-Trac is a step up from the Safari, and there are a lot of options. But if you are just going to compare the Safari and E-Trac on depth, I have found there is very if any difference when using the 11 inch Pro Coils on them. Especially on silver.
The Safari is much easier to set up and use and optimize since there are a lot less functions and adjustments. It does have a slightly slower recovery speed.
The Ferrous and Conductive numbers on the E-Trac do help out a lot, but if you are primarily a tone hunter, then it does not matter much. I have found the Safari actually has a bit of a higher tone on silver and it's easier to differentiate.
Bottom line is, if you primarily hunt for coins and jewelry in parks, fields, dry sand hunt, the 705 will handle those applications very, very well. It has much faster response time to separate trash from treasure, more sensitive to gold using the small 6 inch HF DD, and the Coiltek Digger coil is killer on old silver and copper coins. Plus...it's a heck of a lot easier to swing !
My 2 cents.