I needed one about a month ago and debated between these two. I eventually ordered the Delta mostly because of price, but they miss-shipped it so I went out and bought the DeWalt because I could get it locally and could get the 10% Lowes/Home Depot discount.
Here are the differences and my opinion on each.
I chose the Delta because the DeWalt had no lockable switch. I found that to be wrong. The Delta has the removable red pin. The DeWalt "slap-switch" (easier to shut off) is nicer. It wasn't until I got it and saw there's a hole to put a padlock to lock it. I was happy.
The Delta uses double-edges disposable blades. They cost $25 for a set. They are not resharpenable but I've been told they last a long time. The DeWalt has much thicker and heftier single-edged resharpenable blades. Cost is $35 for a new set and probably about $20 to sharpen (or free if you can sharpen then yourself). They are reported to last longer.
The Delta runs at 8000RPM which is pretty standard compared to others. The DeWalt runs at 10000RPM. Benefits? More cuts per inch? Drawbacks? Louder?
The Delta has a screw lever to lock which allows a stronger lock on the posts. But the posts are closer together which could detract from it's effectiveness. The DeWalt has an easier flip bar to lock, and the posts are farther apart which should make it more effective. Either way they both seem as effective as each other.
The Delta has smaller lighterweight black finish tables. The DeWalt has larger, heavier stainless steel tables. When it comes right down to it the size difference doesn't make much difference. You need tables that are 4-6 longer to be really effective. There are easy ways of doing this yourself if necessary.
Both use sheet steel surface for the planer bed. Not nearly as substantial as cast iron but effective for their size. The Delta uses thinner sheet steel, the DeWalt uses thicker sheet steel.
The Delta has a bar that acts as a handle and a roller to pass wood stock back. The DeWalt has a steel top that lets you rest wood ontop. I actually prefer the DeWalt now that I've used it. The DeWalt also has handles.
The Delta has a plastic hood which cost $15. The DeWalt has a (stronger?) metal hood that costs $35. Price isn't an issue since most places include the hood for nothing. The Delta hood is easier to remove. The DeWalt is a more permanent attachement (screws) but I repalced the screws with screw.
Knobs to make it easier to remove. Planers generate alot of shavings. You need a dust collector for one of these. I'm not sure about the Delta, but you need to remove the dusthood in order to put the beds up for storage.
Both are screamers. The DeWalt is a little louder than the Delta, but it's hard to compare..... try comparing a Boeing 757 and a 767 standing 5 feet from the jets. They are loud. Hearing protection is a must.
The DeWalt is almost 2" taller than the DeWalt, an important consideration if you plan to put it under your bench for storage. I keep mine out on a dedicated steel stand.
The Delta came be bought though mail order for $329 including an extra set of knives and the hood. The DeWalt can be bought for $399 with the same accessories. This is usually the main reason people choose one over the other.
This usually ends up to be one of the deciding factors for many people. Do you like black or yellow. They are so close as planers that we often joke that the only difference is color.
Bottom line: Both are excellent machines for what they are intended to do... surface and finishing planing. You don't try to surface 200BF of roughsawn hardwood in one go with these little portables. You can overtime, but not all at once. The are both well made. The DeWalt is a little heftier (heavier, stronger steel, etc) but many people don't think it's worth the extra 25% in price just for that.
I'm happy with DeWalt. I ordered the Delta and told everyone that I'd just as soon buy the DeWalt if they were out of stock on the Delta. The problem with shipping caused me to put-my-money-where-my-mouth-is. I don't regret it.
Well, I wish I could have bought the $1000 Delta 15" planer, but that's another story ;)