Having run both with a fuel pump for alky, here's my take.
39PWKs:
Good:
Run of the mil, very common, parts easier to get, separate jetting circuits. CHEAPER.
Bad:
More circuits to jet (half empty/half full..
), dump fuel to change pilots or mains (remove bowl)
41.3 Packard Lectrons
Pros.
Simple. Easier/less adjustments. No need to remove the bowls unless you want to clean or change needle seat (gravity to fuel pump, etc. or to set floats). Remove 3 screws to twist and adjust the metering rod for off idle to mid range, twist the PJ for top end. No jets to buy. Less finicky about temp changes for methanol. Can be converted from gas to methanol with a simple switch of the metering rod, and back to gas just as easy.
Cons.
Expensive. Non conventional compared to Mikuni and Keihin. The correct metering rod is 100% critical to get these to work. Need new throttle cable for the bigger carbs (Stock worked fine on my 39's, it would not open my Lectrons all the way and I have an aftermarket throttle). Needles are 35 bucks each or so...roughly. Less circuits to tune (Again, half full/half empty)
I ran my 39 PWKs almost all year. I received Lectrons at the end of the season, and the ONLY thing I changed was carbs.
I felt like I was going slower on the track, but my ETs improved and were much more consistent with the Lectrons. The bike was smoother in the middle RPMs...so it felt slower, although it was going faster.
These carbs were given to me, ran once...and setup for a 10 mil twister for 1/8 mile. So...I had to lean it out just a hair. I called Dennis Packard to see about the needle, he said it will work. Don't drill the carb, don't touch it. Told me how to fine tune the metering rod (same as camatv and another user told me, word for word)
I hit the lotto jackpot with these babies, I know they retail for 700 to 800 bucks, money I would've never spent on carbs alone. I just got lucky....
Fine print would be this. Do I feel I"m missing out on anything compared to my 39's, ET wise, etc? Not really...to be honest.
But I like the simplicity and ease of tuning on these carbs. to be honest, once they're set....they're pretty much set. I leaned out the needle/metering rod half a turn to go from racing the ice at 660' feet to 300 foot of sand down at Gilbert. that's all I changed.
If money is not an issue, or if you get a set for a fair price, get them. You'll love them. But I strongly suggest discussing your setup with a known Lectron Specialist, namely Packard or Fast by Gast.