[Adger Smith]

James,
Yes all cranks flex. How much depends on a lot of factors. Yes an internal will probably flex less when all else is equal. I'll give you a real life example. There are a lot of guys on this forum that have been around long enough to have lived this example. Small journal 327/283 style cranks needed a lot of clearance to keep bearings alive and friction to a min.
Along came big journal cranks. GM added 1/4 in in stroke from 327 to make the 350. GM engineers decided they needed more beef in the overlap of the rod and main. so they added material to the mains and rod journals of the 350 cranks. Instantly the sharp engine guys say no need for those gigantic rod and main clearances. The cranks didn't flex as much so no need for it. Now we have better materials and the clearances are even tighter allowing the use of 0 weight oils. Another thing you will see in the to be heavy or lite on crank weights is what happens at a gear change. The heavier crank can actually act like a flywheel and put some inertia into the pull off the gear change. You have the engine at, say, 7,000 and you make a shift. The shift tries to pull it back to converter stall and the weight of the crank acting like a flywheel tries to keep it @ the 7,000 RPM, but can't so it's effort transfers to the tq converter where the converter does it's job and multiplies torque... see. Now what you have to decide is if that extra Tq/inertia converted at the shift is more important than the few pounds of weight trying to pull back up to the next shift point. Since this is your first, just build and don't get caught up in all the trick of the week science. You will have a steep enough learning curve without working to reinvent drag racing. You will always hear racers talking about lite weight because it is easier to understand the effects, but that is "The rest of the story"...
BTY: You had better read the rules about weight of parts in stocker engines. There is a clause in the rule book about excessive weight removal, even for balancing. Wrist pin and above is where you need less weight. All that weight stops and starts twice with each stroke.