usually below about 100 tds is considered low, but there are people who would still say use RO even down to 50 TDS.
If you've got good pressure, then RO will take your TDS down to nearly zero which will obviously be cheaper than resin, BUT the advantage of resin only is that you can fill your tank super fast.
Personally I'd use resin at first, see how you get on. If you find its costing too much, go RO.