Subdomains or separate sites?

For non-related niches...

[nervo]

I am planning to develop a new website on which I would cover various non-related topics/niches over time. Every topic would be covered with about 10-50 pages. I guess they could be considered as different sites. On each of those I'll feature articles and info, affiliate links and AdSense ads. What I'd like to achieve is to have one domain name with the ability to add other sites/subdomains/niches over time as I find new niches which I'd like to cover.

I've read about the advantages of having directory-like structure separated with subdomains in cases where there are large quantities of topics/content. In my case, I guess I'll have something like 20 different, 10-50 page mini-sites. Also, having keywords in subdomains seem to work well on SERPS - in the past I had success with optimizing a subdomain non-related to the main domain so I guess that SEs treat subdomains as completely separate sites. Also, regarding the sandbox theory, it seems that ten separate domains would have better chance than one with many subdomains.

My question is:
Is it better to have one domain name with all topics in their directories or to make subdomain for each of these topics?
Or maybe to register separate domain name for all those topics and develop/SEO/market them separately?

All your thoughts & tips are greatly appreciated! Thanks much!

[physics]

Seperate domain names are best. Having too many subdomains can be a bad idea as this could be tagged as spam.

[milanmk]

[webmasterworld.com...]

[nervo]

I thought so, thanks...