My source is experience lol. I've had several sets of coils die on me, and they've all been the round coils. Not all are bad though. I've never dug deep enough to find the design difference between the good ones and bad ones, though. Never really cared to..

 

I'm still on all 8 stock "round" coils, so I have no complaints. My 6.0 came with the square ones, and I sold them.

 

I thought I read somewhere where the ls2 style coils, the round truck coils, have something like twice the power of the square ls1 style...

 

L33 and L59 got round coils.

The round coils don't have more power, but the heatsink and internal design allows them to run with much higher stability under high load and stress conditions. Basically, where the square coils start to sputter, the round coils will keep on ticking.

When the square coils get hot, the internal resistance spikes higher and you begin to experience loss of spark at random intervals.

 

Coil comparison

AC/Delco D585

Motor King D585

 

I put cut up pieces of beer cans around mine just to make them awesome!!! No noticeable difference between my 01 sqaure and L33 round, either will knock you off the step stool when bit!!!

 

Well, now I know the round coils do actually have more power output and that's why they're more stable under stress.

 

So first, can I swap the ignition coil itself between round and square? Say if my 5.3L came with round coils, can I chnage them to square if I wanted to? Also, and I know this was kind of touched on before but I couldn't find a clear answer, but which of the two (round or square) are better? I'm not looking for performance per say, or to boost hp, but more which will last the longest and and will work the most efficiently?

 

Round is better. You could change to square if you really wanted to be less reliable.

Spark coils don't boost horsepower above your current best possible, nor do 30mm coil wires with quad shielding and neodymium chokes. The only thing you can get out of good coils and wires is a good burn (but if you're not getting full burn that's just lost horsepower)