I would go for 14.2mm gauge. It is a lot more realistic and is not a particularly exacting set of standards to work to. Apart from if you want to run Tri-and stock I don't see anything to commend the use of 12mm, unless, like me you are modelling Continental TT to 1:120 scale. The standards are basically the same for 14.2mm and finescale 12mm, just 2.4 mm wider. I don't think there's a better scale to work to than 3mm, especially if you are interested in scratch building, dimensions are readily converted, the models are a size where you can see what you are doing without being so big that the cost of materials and the effort of cutting parts becomes an issue. If it hadn't been for problems with the supply of driving wheels back when I was modelling it, I wouldn't have given it up. I think I probably have the world's largest collection of 3mm locos with no driving wheels I keep thinking I should look them out and stick them on eBay. I would certainly go for it, and if it is by way of a small plank to test the waters give 14.2mm a try too. I think you will be very pleased with the outcome. The 3mm Society are a good bunch too, although they seem to keep themselves to themselves to some extent. Their magazine was well worth the subscription in itself.