it all depands on what you want to do.

 FLAC is Fast Lagrangian Algorithm Code, and ABAQUS and COMSOL is finite element based. FLAC fundamentally uses finite difference technique to solve problems. Get the global force to balanced out. FEM of course uses weighted residual and Galerkin way. 

I am from Geomechanics. FLAC is popular in Geo, because it some sense it simulate reality and are able to handle wide class of nonlinear problems, although for some, might question the validity of the solution. COMSOL (FEMLAB) is a nice FEM program for academia, not much mesh and robustness for industry class problem. It has some nice features though. However might need to take considerable learning before you get comfortable in nonlinear problem. 

Abaqus of course, is robust commecial code. FEM based. You might want to take special attention on the boundary of your mesh if you are doing anything dynamics. Nothing bad about ABAQUS, maybe just expansive. If you are student, and wish to join industry after you graduate, it might be a good time to pick up some knowledge on ABAQUS.

Of course, for all nonlinear problems, you will still have to get a strong grib on it before you attemp to solve them using any method you wish.

In term of results, that is up to personal believe. Do you believe in the result FEM gives you anyway? the solution come with certain assumptions and are they reasonable?

I don't track FLAC for awhile, do they incoorporate multicore solver yet? Send an email to Peter Candle and his team to ask them. I think FLAC cost less, and it is more easier to understand in term of the way they solve problems.

I am FEM guy, but I use FLAC too. I think it depands on your problem at hand, and who is picking up the bills.