I'm not sure if this will be much help or not but I had the same dilemma as you a few months back and opted for INK (iLac) and I haven't regretted it. They have a fantastic range of colours which I find gives excellent coverage with only 2 coats. It lasts really well and is excellent value for money. Their Facebook group is very supportive too. Each order I've placed has come really quickly too. And their lamp is fab!
TGB also has a huge range of colours and they seem to last well too from. What I've heard, so it was a tricky choice but in the end the reasons I didn't go with TGB were it seems popular items are often out of stock.
Although people keep saying their bottles are excellent value, as they are 20ml bottles it costs nearly double the price of a 15ml iLac bottle so would be a huge outlay to set up and I'd be worried about less popular colours deteriorating if they didn't get used up quick enough.
There are also other threads on SG where NTs mention poor customer service which I must admit put me off.
Apparently, they will now only sell to new clients if they send proof that they are qualified, however they have stated they will still sell to all existing clients, qualified or not, so it's still a bit vague.
All that said, I was still intrigued by their BIAB (builder in a bottle) as i'd heard such good reports about it. 9 times out if 10 my clients have a hard gel overlay on natural nails and although I usually just file off their gel Polish and infill, I liked the idea of a soak off hard gel so I ordered one bottle of BIAB and their rubber Base coat (which I was advised to apply on top of the BIAB before applying a different brand of gel polish). I have to say I have been very impressed with the results! It really does live up to its reputation and My clients get no Lifting or chipping. I've also used it over fairly long tips and it also lasted really well. It's prone to heat spikes, but I get around this by curing for 5 seconds. Remove hand for a few seconds, then cure as normal and that seems to do the trick.