Chastity and orgasm denial discussion

Hi all. Sorry if this thread isn’t the correct place for my query. I’ve been following chastity with interest and feel it’s time to commit and buy a device. Once I get used to wearing mine, I’ll pop by the lost-and-found section looking for a keyholderess who can tease me, torment me, and keep me begging until release comes. But first things first: the cage itself. Being new to these gadgets, I’ve chosen to start with an inexpensive Chinese copy. That way, if it doesn’t suit or fits wrong, I only lose £10-15 instead of over £100. My options have narrowed down to a CB6000 clone or the CB6000S version. The main difference seems to be tube length: 3.25 inches for the standard model and 2.5 inches for the S. I’m unsure which method I should use for measuring. Measuring from the tip along the top side of my shaft gives me 3 inches, but if I measure underneath (tip to scrotum), it’s 2.5 inches. Which measurement is correct, and consequently, which model suits best? Thanks in advance.