mckinneymke,
The 4Point11 has an effective mass 18gm which would be great for the Koetsu. If you can afford it this is superb - I have set one up with various cartridges including Van Den Hul Calibre & Kuzma CAR50 - compared to Moerch DP8, Helius Omega on the same TT it was a class above.
The 4Point9 is 13g - still ok for the Koetsu - its medium mass.
I think both arms give you more flexibility than the Groovemaster.
I have a truckload of arms, and you cant beat super high quality bearings and quality of design, ease of set up which gives you better results, rather than just focus on mass.
If you want a very high mass arm I'd find a mint Fidelity Research FR64S, silver wired, with B60 base if possible, over the Groovemaster. There are some polarising opinions on the FR on these forums, but they are very synergistic with Koetsu's and low compliance cartridges.
