Both are great for isolating the lower lats but which is better because of its increased range of motion and time under tension.
A classic exercise, the straight-arm pulldown is a great pre-exhaust movement for lats or as a finisher at the end of a brutal back workout. It completely isolates the lats while also calling into play the core musculature. You have probably felt your deep abdominals hard at work during a gruelling set of these pulldowns, and that is absolutely critical and expected. The exercise requires considerable stabiliser activity, especially the heavier you go. If you are stopping the bar on the upward phase when your arms approach parallel, your lats and core are under extreme tension. Many athletes allow the bar to travel too high, causing a loss of tension in the lats. The two keys for this exercise are to hold the peak contraction for a count at the bottom with the bar directly in your line of sight, and pulling it all the way into your body at the bottom of the rep as you squeeze your lats.
Decline Cable Pullover
This lower lat exercise takes a little trial and error in terms of bench placement. You have to place the bench far enough away from the stack to allow your arms full extension at the start of the rep without letting the plates touch down. You might have to do a couple of trial reps to ensure you are set up correctly. Unlike the straight-arm pulldown, the decline pullover requires minimla stabiliser activity. If you are thinking the pullover is a chest move, you are correct, but it is even more so when performed in an incline or flat position. The decline pullover is an excellent lat exercise whose key is a shoulder-width grip and completely straight arms throughout the movement. Opening and closing the angle of the elbows will automatically involve triceps and chest.
The Winner: Decline Cable Pullover
Even though your head is lower than your legs, with the cable gravity does not diminish the effectiveness of the exercise as you pull the bar into your lower body, therefore your alts remain involved longer. Because the bar travels closer to your legs, you can squeeze your lats as you would during the straight-arm pulldown. However, the angle of your body causes the lats to be engaged through a greater range of motion. In contrast to the straight-arm pulldown, which requires you to stop the bar to keep tension, the decline cable allows you to reach back overhead while maintaining that necessary tension in the lats. The fact that you do not have to worry about balance makes the decline cable pullover the clear winner in the race towards building incredible lower lats.