Personally, i never suggest SLI.
The problem is that too many things are a question mark when it comes to SLI.
Let me explain:
SLi dosent work on every game. 1 negative
Drivers are even more prone to failure. 2 negative
Produces more heat. 3 negative (dont forget that this can create heat bubbles, thats really bad).
FPS are more variable (gives the impression fps are lower than they really are). 4 negative.
Needs a lot of power (PSU). 5 negative.
THats 5 negative points. Normally, SLI will outperform a single card that costs more, but this is mostly in very high end.
If you are looking at 2 550Ti, look at same price point for a single card (and see if its acceptable for you)
Also i forgot 1 thing, SLI must be working with 2 gpus that are exaclty the same. If one card has even a little more problems than the other, you will notice that.