Noslepums, My post above was written under the assumption that the 12" SME arm you are contemplating is the SME 312S, though you wrote "312". The 312S is a new generation magnesium arm based on the SME IV. It differs from the V-12 by having a removable headshell (convenient for cartridge swapping and azimuth adj.) and by also having dynamic VTF adjustment. For me, the latter is more important than the former, but for others, maybe not. I'm not familiar with the 312.
Albert Porter has the 312S arm and is getting a V-12 for comparison. I've never heard the 312S. I agree with everyone else that the length of your arm should be of a lower priority to others for getting started. Set up what you have and enjoy some records. Then go from there.
If you follow Raul's advice for a nude SP10 with an arm on an arm pod, then switching to a 12" arm later should not be an issue besides adjusting for the new alignment. Have some fun.