Flat or ready folded......which is better for a beginner....

What is flat? Solid wood stock? Much better for beginner, folding stock for coolness. Or you asking about Khyber Pass stock?

Do you have a press? If yes, a flat is an option. If no, then not really. I did my first flat with a self-made jig from steel square tube and a BFH. Not fun. Next one incorporated a large bench vise. Easier, but still not fun.

Once it's in shape, the rails are the only hard part if you don't have a spotwelder. I drill a series of holes and plug-weld with a MIG. Without a welder of some sort, you may as well not even bother.

Flat with a proper folding die would be much better for a first time imo.

Be aware that  some folded blanks that are being sold do not heat treat the same way as 4130.  4130 is a no brainer, use a map torch to heat it red hot, keep it red hot for a minute and quench in water.  Yes, 4130 is a water quench alloy for parts of this thickness.  Put it in the oven for 1 hr afterward at 450-500 degrees and your done.  I've read on the forums that some guys have had problems with the folded blanks since the alloy is 1050 and it doesn't heat treat the same.