Neither. Hanging leg raises.
The function of the abdominals is to bring the rib cage towards the pelvis. If you want to target your abs, it has to be a crunching movement. Other exercises indirectly work the abs by making them contract to stabilize, but they won't work the abs through a full range of motion. While standing, raise one of your legs in front of you while placing your hand on your abs; you won't feel them doing much because that's not their primary function.
If you want to target the abs, do crunches. Side bends and adding twists to your crunches is all you need to make your obliques tight. Do leg raises and other exercises like that if you think you get something out of them. Many do...
I used situps to strengthen my hamstrings in high school, it worked...
I can't remember when I used them last to target abs, 7th grade maybe before someone showed me different.
I use leg raises as an annicilary to crunches and side bends...also, leg raises help to strengthen the area around my L5.
Do what works. Just remember the "blowin' out the match technique," regardless.
IMO, it really doesn't matter which exercise you use...as long as you feel them and hit them from every angle. For example, for me I feel that decline crunch (upper and middle abs) and V-ups (lower abs) are better alternatives than crunches and lying leg lifts, respectively. That's just me.
What makes them show is bodyfat loss anyway.
I find the big inflated exercise ball a good place to do crunches too, and yeah burning fat is also key so I have been trying to do some HIIT cardio 2-3 times a week on my non-leg days.
Abs are very easy to train. You only need a few movements to develop them to the point where they look good. After that it's just a matter of how lean you are. I'd rather have a tight midsection than try to make my abs more massive. That is unless you want that bulging abdomen look that seems to be so popular in modern professional bodybuilding! I hear HGH helps with that!
Also, I don't recommend doing any weighted obliques exercises, such as weighted side bends. If you get your obliques bigger, your waist looks wider. Also, serratus development REALLY adds to that quality look. DB Pullovers are the best exercise I know to hit the serratus. And I mean lying perpendicular to a bench with only your upper back touching the bench (you know what I mean). This way, you can drop your ass and enhance the ROM and stretch you get, as opposed to lying on a bench in regular fashion, where you can't drop your ass, limiting the effectiveness of pullovers (in my opinion).
dont ever do cruches, weighted decline sit ups here and there.
I like leg raises too, but I do mine a lil different then most.
I hold my self in the "up" position of a a chin up & bring my knees to my chest using my core to stabilize in a slant position....
need to do more pull overs.. id like to to have some bigger/defined serratus. I rarely do them on chest days.
Both movements incorporate hip flexor activity that can minimize the amount of work the abs are doing and hanging leg raises and bridges are much better substitutions.
When you perform activities that need core recruitment. How does the core contract? Is it concentric or isometric? It's isometric, so I would ideally recommend doing isometric abdominal work like bridges. You'd really need to train the obliques hard too though with side bridges or suitcase deadlifts.
Former Marine, UT-BSN, NSCA-CPT, NASM-CPT, CSCS
Current lifts: Squat: 545 | Deadlift: 600 | Bench: 425 | P. Clean: 325 | Press: 285