It definitely a case of personal preference. But I prefer the Ranger/Rogue MC over the Fighter/Rogue MC.
Why?
1. Rangers wearing light or no armor get ambidex and dual weilding free. Rogues will always be wearing light or no armor, so this is a good match. Rangers wearing light armor will require a high DEX stat to get the most out of that leather armor and the best AC possible. Ya know what? Same is true for a rogue!

Perfect sinergy.
2. Rangers can take the Hide, Move Silently, and Search skills for 1 skill point. A perfect compliment for a rogue mix. When taking a ranger level, take a mix of these skills. When taking a rogue level, take the theiving skills (i.e. open locks, disable device). Also consider taking a some points in wilderness lore when taking a ranger level.
The skills available if upleveling as Fighter for a Fighter/Rogue mix are more limiting. However, even a 50/50 ranger/rogue mix can still have a solid skill scores in the stealthy and theiving skills.
3. Are the extra feats that fighter levels (instead of ranger levels) worth a fighter/rogue over a ranger/rogue? Matter of personal opinion. What's more valuable to you, the extra ranger stealthy skill points or the extra fighter feat points? IMHO, the extra skill points from a ranger/rogue allow you to build fully functional rogue that you wouldn't get from a fighter/rogue. Of course, some of it depends on the ratio of the mix. If you take 10 levels of rogue before taking any fighter levels, you'll be OK with your skills. Except that during all that time of getting to level 10, you were a pure rogue. A 50/50 rgr/rogue (with a good INT) can probably maintain the same level of skills and gain the improved fighting skills that those ranger levels bring.
I'm playing a 50/50 ranger/rogue in my current 4 person party. To this point, she's an awesome stealthy killer. I'm playing her as more of a moderately high STR tank, than a finesse rogue (played finesse rogues in my last 2 parties). Her only "weakness" in her stats is that I stuck her with only a 10 WIS, so she can only cast a single level 1 ranger spell. This doesn't bother me, cuz if I wanted to cast "ranger" spells, I'd have made her a druid. I wanted a higher STR, high DEX "tankable" fighting rogue for my party of 4 and something had to give.