I dont think there is a right or wrong answer to that.
I suspect the product or service your are offering would have a bearing on the choice.
It would certainly be easier and more cost effective to have one main portal to market than a number of small sites.
Of course if you want to appear as seperate companies then choosing the nsmaller sites option would be best.
You also have to decide what visitor your trying to attract and what you beleive their prefences would be.
For example a recent servey i saw suggested the over 40's tend to ignore the "small" sites alltogether and are only drawn to "large" corporate looking sites(apparently this gives them the feeling of security)wheras you and i know anyone can build an impressive site and still be a rouge!

If the portal content would be all on topic then I would go that route. If it is going to have multiple topics then I would split it.
A prime example of this is the wales site I am building. It has
weddings in wales
Stay in wales
chat in wales
ancestry in wales
eat in wales
sport in wales
Whats on in wales
Animals in wales
date in wales
party in wales
and a few more as well.

Now these are going into a portal as the overall theme is 'in wales' this is ideal as people using one section will gladly use another as it is part of the service (as people use Gmail because it is part of google). I thought long and hard about whether to go down the separate site, or down the main portal but with subdomains. We are going with the latter so we will have
stay.inwales.eu
animals.inwales.eu etc
but ALL accessed via the inwales.eu portal.

Originally Posted by midlandi

For example a recent servey i saw suggested the over 40's tend to ignore the "small" sites alltogether and are only drawn to "large" corporate looking sites(apparently this gives them the feeling of security)wheras you and i know anyone can build an impressive site and still be a rouge!

This is an excellent pointer ( I mean the over 40s stuff), I will bear it in mind next time I advice customers.

I was actually looking at Portal V mini sites from the view point of search engine optimisation.

I know promoting several sites is more expensive than promoting one single site however like OWG indicted in the post below, you can target particular sites for particular products for example we have a golf equipment site, one site is optimised for junior golf equipment and a second site is optimised for smaller golf items like golf gifts.
Now we need a 3rd site to target Irish customer (because they prefer to pay in Euro), I still cannot decide whether to create a completely different site for Irish customers or just create a folder and put prices in Euro and optimise that sub pages for the Irish.

Originally Posted by OldWelshGuy

Now these are going into a portal as the overall theme is 'in wales' this is ideal as people using one section will gladly use another as it is part of the service (as people use Gmail because it is part of google). I thought long and hard about whether to go down the separate site, or down the main portal but with subdomains. We are going with the latter so we will have
stay.inwales.eu
animals.inwales.eu etc
but ALL accessed via the inwales.eu portal.

I think the above option is excellent for many reasons including the fact that you can create separate ftp account for the sub sites for different webmaster to manage each site., also, you save money in not having to buy separate domain for all those topics but on marketing, each of the sub domains are regarded by Google as separate sites so you still have to put in the same amount of work to get them ranking well as you would put in a new domain.
I'm I right or I'm I wrong ?

Right and wrong. Google look at sub domains as a different domain, but also grants an association with the sub to the main. Yes you have to market them independently to a point, but the benefit you will gain from shared access by members of the main portal well outweighs the extra effort.