single ND filter vs stacking two ND filters together?

I'm looking to get two ND filters but im not sure which route to go with. I was wondering if there is a degrade in quality or increase in vignetting if I stacked two ND filters together? I'm looking to go with Singh-Ray solid ND filters paired with a Cokin Z Pro holder.

My options are:

Use only 1 ND filter at a time
3-stop filter $160
4-stop filter $250
Total: $410

vs

Stack ND filters together
1-stop filter $160
3-stop filter $160
Total: $320

If you get good, quality filters like Singh Ray or Lee, you shouldn't have to worry about any problems.

On the other hand, the more surfaces and the thicker the substance is that light needs to pass through, then more you are degrading the quality. For occasional use, then I see the advantage in multiple filters, however if you need it often, then invest in one good filter.

perfect. do you think 4-stops is good enough for f/1.4 on a sunny day? i plan to use a strobe to light the subject

edit: https://photography-on-the.net …/showthread.php​?t=1154254 i'm the OP of this thread. the first photo shows a 135L @ f/2.0 with 3-stops ND filter. I think it was a light overcast day as well. What stop range should I be aiming for if I want to use f/1.4 on a sunny day?

double edit: looks like 4-stops may not be enough... may have to go with the 3-stop + 4-stop and stack em together. if its a little too dark i can always increase the ISO... right?