Whether 2t9 + 2t10 > 4t10 is very dependent on the pieces involved in your T9 and T10 sets. With higher ilevel T9 pieces you'll need better T10 pieces to justify switching bonuses. It's best to test your gear on
FemaleDwarf.com or the spreadsheet to see when it makes sense for you to transition. In general for SV specs though, the margin required to make 4pc T10 better is much lower than it is for MM.
If you never got 2t9 at all then you probably just want to go straight to 4pc T10, however if you will be awhile saving up the emblems for it, and you have extra triumph emblems, you may be best off temporarily picking up T9 for 2 of your offset slots (usually pants and helm), depending on if it spreadsheets out to a dps increase for you.
Death's Choice performs much more favorably for SV than MM, but Deathbringer's Will is still a strong trinket for SV spec. However if you get a Deathbringer's Will you may have good enough gear at that point that MM may perform better for you. To get a rough idea of comparative trinket dps use femaledwarf or the spreadsheet.