a 600ppi file will not produce a better print than a 300ppi file if the output is 240dpi?
No. It won't At 240 dpi you could use a file at 100ppi and you would see no difference. (But probably you mean 2400 DPI so keep reading)
Agree with DA01, the 600 PPI on a photo has no sense.
Let me explain a bit some of thoose numbers:
Print for offset printing - 300ppi
This is relative. It depends on the real output of the screen used.
If you are using a screen of 150 LPI, which is a standard for a normal magazine, for example, you use double of that... 300ppi. But that screen can vary depending on which paper you are using. If you use coated paper its 150lpi (300ppi) but if you use non-coated paper it can be less (for example 133lpi - 266 ppi)
Actually, that proportion LPIx2 is not entirely correct, because you could go for a Square root of 2. or LPI x 1.414 or just LPIx1.5
This will give you some margin to work like 212-300 PPI.
Digital print is very board concept. In this part, I'll refer to High volume digital print.
If the output quality is similar to offset print, yes, use the same number... 300ppi.
Some digital printers do not use a screen, but rather an error diffusion approach (stochastic) in some cases, in some parts the borders, are sharper than using a standard screen.
You could use 200ppi... What! lower resolution than in a magazine? NO WAY!
Actually, 200 PPI on a photographic paper has more resolution than a 300ppi photo on an offset magazine for the reason already mentioned. Those 300 PPI are transformed into 150 LPI This is 150 points on information. 200ppi on photographic paper are not transformed, they are actually 200ppi on the paper.
This is closed related to the LPI output.
The explanation is a little offtopic here, but If needed I can explain further. 150 LPI is based on a 2400 DPI plate or negative. For Line art you can use sub-multiples of that number. 1200 PPI or 600 PPI in some cases.
This 2400 DPI (laser dots) are used to produce a halftone dots, normally in a proportion DPI/16. This way you can produce 256 shades of gray (16x16=256)
High quality art print
You can go to 400ppi on a 200LPI print. Of course you need a high-quality provider.
The resolution of images and backgrounds depend on the viewing distance.
(The unit on this image is wrong, it should read PPI)
This table starts at 300 PPI at 30 cm. Double the distance, half the resolution.
Try to keep your texts in vectors so you have them, resolution independent.
PPI are totally irrelevant. Only see the real pixels.