So are you arriving on Thursday the 4th or Friday the 5th? DL is open until 8 on the 4th and midnite on the 5th. That alone tells you that DLR will be more crowded on the 5th vs. the 4th. DCA is only open until 6 on the 4th and 9 on the 5th.
If you are arriving on Thursday the parks shouldnt be too crowded at all.

I do just the opposite of PP. Well go into DL first and grab a few FP to use later in the evening. Then cross over to DCA and do some rides there. Cross back over to DL later to use your saved FP. This plan works best if you get there in the a.m. but it can still work in the afternoon as well.
If you are arriving on Friday I would still do the same thing, its even more important. On Friday, DLR gets more crowded in the evenings as us locals start arriving after work and school.
Thursday youll still arrive in time see some entertainment. CELEBRATE at 7 p.m., Aladdin at 4:15, HSM 3:35-3:55. If youre arriving on Friday, youll be able to enjoy more entertainment. CELEBRATE at 3:30 or 6:30, fireworks at 9:25 (its why many AP go there on Friday evenings), Pixar Play Parade 5:15, HSM 3:35-3:55, Aladdin 5:50.
You can definitely take the train to get an overall view of DL but you really won't see alot of the park from the train. You'll see the back sides of many things (though the Grand Canyon and Dinosaurs are pretty cool!). Another way to see parts of DL is to ride the monorail. You could even enter that park using the monorail.
Do some rides, see some entertainment, enjoy the sights and sounds!
While there don't overlook some of the overlooked attractions. Don't know how old your kids are but my DD is 10 and she loves to spend time at Redwood Creek Challenge Trail in DCA and the Animation Academy in Hollywood (I enjoy this too!). Do the drawing class and see Turtle Talk with Crush. Grab your free tortilla at Mission Tortilla Factory. Bread at Boudini's. It it's warm out, they can play in the water areas at DCA in Flik's Fun Fair area.
As far as is one park better than the other in the late afternoon, early evening. My answer, especially on Fridays, is yes! DCA is almost always less crowded than DL.
Have fun!