If you can afford it, it is probably best to use Arimidex but have letro on hand in case you have Arimidex doesn't adequately control the bloat or incase of gyno that you aren't able to control with Nolvadex.
Why not just use letro? Well, letro seems by far the most likely to damage your lipid profile. Arimidex is a very good drug. If it does the job, then I would stick with that. For example, the vast majority of HRT patients that need estrogen management use Arimidex, followed by Idolplex+DIM, not letro.