Should I use Silver Lance+ or Brave Lance on Effie?

Between Brave Lance and Brave Lance + there is only a difference of 3 attack, it is not too much and you don't have any unit with a Brave Lance +. So if you want to give her Brave Lance now go for it, or you could wait until you get a unit with Brave Lance + that you don't want it.

If you are using Death Blow and Wary Fighter (her basic skillset) then Brave Lance will still be better than Silver Lance+. Assuming she is 5*, has neutral IV's (meaning 40 atk) and is attacking (+6 atk from Death Blow);

Brave Lance (5 mt):
40+40+5+5+6+6=102

Silver Lance+ (15 mt):
40+15+6=61

She is too slow to run anything besides Wary Fighter in her B slot to stop mages from killing her. With a regular Brave Lance, she will deal 51 more damage on attack than a Silver Lance+, and lose only 10 dmg when defending against an attack. Although 10 dmg is still a lot, it is not nearly as much as the potential 51 atk she can miss out on without a Brave Lance.

Yes, Brave lance will be better if you are running the typical brave build with Deathblow. Silver lance+ out performs the Brave lance if you are going DC. Though, Brave lance with DC is still fine because Effie has alot of attack to work with.