Welcome Johnzon,
As a general rule 240z tend to get better prices. Well that is the advise you'll see on a lot of forums however over the last couple of years I've noticed 260 2 seaters selling for equally high prices which does'nt surprise me at all seeing they generally look identical to me minus the rear lights.
Personally I would say if you would like to buy an S30 with the aim of getting it original factory looking then a 240 is a smart choice. If you just like the curves of the 2 seater S30 then go for either especially if you plan to make changes or customise the car.
I've got a 74 2 seater 260. I don't think of it as an investment nor am I overly concerned about keeping it too original although I like the idea of sticking to an L series and trying to keep within the spirit of that tough early Z look. Also I preferd the 240z rear lights so I just swapped my 260 lights for 240 lights.
Having said all that I know it's getting harder just to find a half decent 240 or 260 2 seater these days, but they do come up occasionally so just keep an eye out on all the usual places, trading post, carsales, NDSOC website, Ozdat etc.
I do know of a member here that has a very original 240 that has sat under a tarp for over ten years. I actually checked the car out for him seeing he was overseas when he found the add so he basically bought it over the phone and since then due to unfortunate circumstances he has'nt had the funds to start the resto. Try to PM him if your interested, I believe he goes by the member name of RACINGSTRIPEZ from memory.
Good luck with your search anyhow and remeber alot of members here tend to post up any good finds on the classifieds section to assist people like yourself to find a car.
Cheers
Sulio