I came to the party very late, and only have the one 25, the 25A. It was my first modern bandai valk, and I liked it so much that I bought an Isamu 29 when it came out.
I initially resisted the temptation to buy more Frontier valks (found the F and RVF at very good prices), and now I'm glad I did. I feel no need whatsoever to acquire any more frontier valks. The main reason for me is that they all have that exact same weird torso transformation that I find annoying (I like the design, but hate bandai's engineering).
I might get a renewal 27 if they ever come out with a different color, but that'll be strictly to appease the completionist in me that wants at least one of every Valk type.