which is is mostly better. The main advantages of the i6 over the T6 are..
Better protocol AFHDS2A which supports telemetry although not in the most useful way.
Available receivers: park flier iA6, near full range iA6B - supports telemetry and CPPM, iA10
Programmed failsafe setup from the TX
Diversity antennas on the TX and RX giving more reliable range
Low power consumption (4 AA batteries vs 8 in the T6)
Compatible with older AFHDS receivers as used by the the T6
It's programming features are otherwise similar to the T6.
Cons compared to the T6 are a more toy like look but it's lighter and feels solid
Cons in general are that AFHDS2A is only supported by the FlySky/Turnigy i6 and i10 transmitters. There is no module allowing you to take the receivers to another radio (at least for now)
Normally I would suggest the i6. Its a good out of the box package However probably the best deal right now, for a limited time/quantity, is the 8 Channel Turnigy 9X mode 1 from Hobby King for $40. If you are not afraid of a bit of modding its probably the best value outside used gear.
It's an easy switch to make it a mode 2. Only seems to be the US West warehouse and I don't suppose this price will last for long. It's usually about $60-$70.
The 9X uses the same AFHDS protocol and receivers as the T6 which are not that great but with a small mod you can replace the TX module with a FrSky XJT or DJT and get a much better radio system. There is also way to upgrade the firmware to use er9X or OpenTX easiest using the smartie board. You will want to upgrade the battery to say, this LiFE for which you will need a LiFe charger. You can do this directly but its best to add a polarized connector since there is no reverse polarity protection. Lots of info out there on the FlySky/Turnigy 9X if you are interested.
Usual caveats about cheap stuff. Quality is variable so you are more likely to get a bad one. However often the issues are easily fixable especially with the 9X which has an enormous user base and lots of hacks and mods.
It comes down to best out to the box vs price and doing mods.
A TGY 9X for $40 (current limited time sale for a mode 1 from HK US West warehouse) is a pretty good deal if you are up for doing the mods and/or were thinking of getting one. It's usually $60-$70 at which price the value proposition a bit less clear. It could still make sense at the usual price as something to incrementally improve or if you know you need 8 channels for the lowest price.
If you are not up for mods and all that incremental additional expense then the i6 for around $55 is a good value out of the box. It's only 6 channels vs the 9X's 8 but all reports indicate the new AFHDS 2A radio system is more reliable than the old AFHDS that the 9X uses.
"Better" really depends on your perspective. However either the i6 for $55 or the 9X for $40 are better than the T6 for $55 since the T6 uses the old AFHDS protocol and has much less modding potential than the 9X.
I have the fs-t6 and have been very happy with it thus far. No range issues, the 20 model memory is great, easy to program and select between models, I can buddy box with the hk 6 ch v2 tx and i use it on my laptop for airplane and heli sim. Great buy for the $ spent. I also have it bound to to hk v2 receiver. I believe Turnigy uses the same protocol as well so it leaves some flexibility for shopping around.