I only have two 31's, Mirage and Hayate. I only really double up with the Hi Metal R's - I have 3 of the Messer VF1J Anniversary valks (picked one up for £25 at Nakano!), 3 of the VF2SS, and I suppose if you take into account all the recolours, 6 VF1's (Max, Miria, the Messer colours and a Fokker version). Much as I love the larger Valks, the HMR figures are like mini-masterpieces, just a fantastic size for display.
It's nice to have these on display in my office though