For me it's pretty easy. Protoform SRS-N in relatively low/med traction and where I want lots of steering. The P37-N in higher grip or where I need more stability. I've also liked the xceed spark but it doesn't seem to work at all tracks like the SRS or P37 does.
Serpent Viper 977e, Serpent Project 4x, Serpent 811T-TE Truggy, Serpent 811e 2.1, Serpent SRX2-MH, Serpent SRX-2 RM, Serpent SRX-4, Serpent F110-SF2, Serpent F180, Serpent 748 Natrix WC, Serpent Cobra GT 3.0, Serpent SRX8, Serpent S811 2.2, Baja 5B Flux, Atomic AMZ 1/27th, Schumacher GT12 SS.
Sanwa, Ielasi Tuned Novarossi Racing Engines, Tekin, Hobbywing, Muchmore.