Welcome on board!
As for which kit you have made the most important choice already by deciding to get a PicoScope
Its then a 2 step process - decide between 2 and 4 channels. You can do a lot with 2 channels, but if you are serious about improving your diagnostic skills then you may regret not going for 4 channels - it saves time (eg looking at 4 x ABS sensors at the same time rather than 2 and then 2) and is great for things like checking timing (cam, crank, injector & ignition).
Of course cost is always an issue which is why we introduced the stripped down 4 channel starter kit - its the same price as the standard 2 channel kit but can be upgraded over time by adding in the missing accessories.