I've built and raced both, so I'll give you my opinion.
Build quality is very good and the kit is easy to put together. However, to do it properly there is a bit if work required to get the front and rear suspension working without binding. Nothing more that some polishing and a bit of reaming on the arms - pretty standard on most kits. The included components are very good, such as front CVD's, recevier pack, receiver switch plug and aluminium wheel hexes.
The car is very strong an stands up to racing quite well. There were no real weak points that I have found so far, other than perhaps the rear body posts which can be affected if you flip it on its lid (which I do, a lot
). However, in order to be competitive there are some hop ups which are almost essential. The ED Transmission is required in order to give you a better drive ratio. You'll achieve greater acceleration without affecting top end. If you want to tune the front end, the Speedshot front end will give you roll centre adjustments that can help with the cars natural off power understeer.
Team magic recognised the shortcomings and due to much feedback from racers, released the G4+ which is a limited edition model. This model incorporates the ED Transmission as well as the Speedshot front end, 4mm chassis, new arms which can shorten the wheelbase, and a new rear sway bar arrangement similar to a Kyosho RRR (ie a V shaped blade). The car had an immediate impact and won the European Championship. Luckily, all of the parts required to convert a G4S into a G4+ are available on line and in many LHS's.
Parts availablity is good on-line, although I believe there may be some local distribution issues in the US which many people attribute to Trinity being the distributor. However, if you a re US based you can not go past Raceday Hobbies. Great service and price.
Build quality of the NT1 is exceptional and did not require a lot of tweaking to get the car built properly. The diffs can be a bit tricky due to the use of a very thin o-ring, and the carry handle has a tendency to want to tweak the top plate. Apart form that, car goes together very well and the drivetrain is incredibly smooth - more like an electric car than a nitro. Some gripes about the quality of components, ie no front CVD's, no aluminium wheel hexes and the shock bodies are composite and not alloy. The standard foam bumber is also very soft and many opt for the hard bumper (which is very hard). However, the only real upgrades required are the front CVD's although being Xray they are expensive. The lack of a switch plug annoyed me but was no biggie.
On the track the car was very impressive. I was able to better my lap times over the G4S on the first day. The car had more steering and felt more stable through the corners. Whilst I am nowhere near the fastest at my club, the car gave me a lot of confidence and allowed me to concentrate on keeping a good line rather than just trying to stay on the track.
Parts availability is good where I am (Australia) in both LHS and on-line. Of course, the cost of Xray parts are up to 100% more than other brands, although it is hard to fault the quality. So far I have yet to break anything, although I have managed to push the exhaust holder onto the front pulley which has required a replacement.
In terms of results on the track, the G4S has achieved 2 European Championships as well as a host of country championships such as Spainish, British, Peruvian and Portugese. Perhaps the car is better suited to the high grip purpose built tracks in Europe? The NT1 has been gaining a lot of popularity in the US and has performed well in every event it has competed in. Not too sure of any major wins but I'm sure somebody will point them out.
My recommendation though would be the NT1, although be prepared for increased runnings costs due to expensive parts. The kit will cost you more especially as you need to buy the CVD's, although when compared to a G4S with ED tranny the price becomes more comparable.