What you want from the gauge is going to determine which one to buy. If you can get by with just one temp, the M4 will suit you. You could get the M4 with two sensors; an EGT and a water temp. Use the EGT during practice to monitor the carb settings and use the water temp during the race. Just having one temp seems questionable. I prefer having two. Maybe some shifter guys will help out and post their gauge configurations.
How do you plan to analyze the data from each of your track sessions? Reading it off the gauge is useful, but you would be missing out on the ability to study the data at a later time with Race Studio. That option comes with a price as the only method I’m aware of is with a Data Key.
Neither gauge will get you the ability to create a track map in Race Studio. It’s a nice feature and you can live without it. With addition of an E-Box will add to temp inputs, a wheel speed sensor and an accelerometer. An E-Box with an M4 will give you three temp inputs, wheel speed, a lap timer and the ability to create a track map within Race Studio. The E-Box comes with a Data Key.
Another option would be an M4-2t with a GPS unit. This will give you two temp inputs with the gauge and from the GPS you get another lap timer, lateral G-forces and the ability to track map. Now you can generate a map of each lap run and that can be compared with your lap times.
There two AIM web sites. http://www.aim-sportline.com/eng/index_kart.htm#mychron4-2t is the site that has sample gauge and equipment configurations. The other is http://www.aimsports.com/products/mychron4/index.html
As always, talk to you local kart shop and AIM has a great support staff to help you out.