Hi Mate,
As an owner of 2 XT70s over the last three years and a recent owner of an XT705, i hope i can offer you some insight into your dilemma...
First of all, my first XT70 didn't die, it was "permanently borrowed" by a "mate" who has subsequently disappeared (doh!). I take my XT with me, broken down in half, in the boot of the car everywhere i go. It is an awesomely capable machine that has never once given me any trouble, and when it tells you some thing's down there, some thing's down there. Its easy to use straight out of the box and practice with its many optional features only multiplies your chance to find decent targets and learn to ignore those not worth digging. Also, it has paid for itself several times over. I rarely walk away from any site, be it beach, playground, old homestead, etc, empty-handed. I literally don't remember the last time i used it and found nothing at all.
I've recently (2 weeks ago) gotten my hands on a 705 and while i will be posting a more comprehensive report as to the differences, the bottom line is if you can save a hundred bucks or more on a 70 vs a 75, you should do it. Apart from the backlight (a long awaited addition - i hunt playgrounds and such at night regularly and have to use a head torch to see the target ID) and the alleged extra functionality on the beach, which i'm yet to confirm in practice, there's really not that much difference between the two.
If this is your first detector, i would recommend the XT70 as a starting point. you can always sell it on ebay or wherever for almost it's new price and go up to a 705 but in my humble opinion, if you're just starting out, or upgrading from some marginal detector, and don't detect wet sand and/or at night, you won't use the new features of the 705.
Cosmetically, the two are similar, with the crap "stand" provided by the original xterras identical in the new series, meaning that pretty much every time you put it down to dig, it will fall over sideways with the control box hitting the ground. i'm curious how minelab "listened" to customer demand about the backlight but failed to address this, despite there being 5 or more after-market stands being made so the damn thing won't fall over every time you put it down. i use aftermarket stands on both of my units.
I'm sure i'll get flamed for making a comparison between the two units after only having the 705 for 2 weeks, but there's a few other little things that annoy me. One of them is that all the buttons and menus have changed from being in plain english to being symbols. Its fine for me, because ive been using the XT70 for 2 years and in the dark, but a new user might find them confusing at best. I presume this saves them having to make different software for different countries but in practice i've found it to be a pain in the arse while changing settings, wondering what some particular symbol means in english.
In summary, both machines kick serious butt, and performance is roughly equivalent, although beach performance on the 705 according to the only videos i've seen, sponsored by minelab, seems to be improved. and it's got a backlight, but if you can save 100 bucks or more on an outgoing 70, spend 10 bucks on a headtorch and aren't exclusively prospecting beaches, i say go for it.
I'll be taking both machines and sample targets to a nearby beach in the next week or two and will report back more comprehensively here.
Good luck and happy hunting, there's a whole community here to support you with any questions you might have.
cheers
joe
PS dont be tempted by "cheaper" detectors from other brands or sources..i'm not a minelab dealer or affiliated in any way but ive tested supposed "equals" from many other companies, they're all a recipe for frustration and tears ;]