I was also using the 640g and am now using Tandem Slim with Control IQ which is linked to the G6, it has been an eye opener using this pump, simply because it does my basal adjustments automatically, and since using this pump my levels during the night have improved massively, so I stay in range during sleep now, no hypos, good quality sleep impacts on mood and daily glucose control, so of great benefit. Only downside is that I like running and I find even in exercise mode which I set an hour before heading out, that I can often go hypo so am having to eat carbs before evening runs, minor issue but I run with others so hate slowing others down if going low.
I am also experimenting using sleep mode 24 hours a day which keeps a tighter range and this has again improved my daily time in range by 10-15%.
In your shoes i'd take the tandem slim and if you can self fund then get the Dexcom G6 at £159 a month, for me it's been the best switch in all my time as a t1 which is about 9 years now.