Welcome to the Q&A Forum

(because of the 404 error pages being constantly found in our pages)

Think of it this way:
Which is better? Re-routing traffic when it's congested or putting up a road block to back up even more traffic?

Yes, it's more work to do the 301 redirects but if you have "pages being constantly found" you should give instructions to spiders to take the different path.

Now, if you are talking about an error such as:
/brand//samsung/13 SHOULD go to
/brand/samsung/13
Then you could EASILY solve this with HTACCESS redirects. I wouldn't risk telling a spider to ignore /brand// because it might have adverse results.

I use the ranking tool quite a bit (It's well worth being a pro member).

What is nice about the MOZ tool is that it checks the keyword phrases you are promoting and shows you your rank, no matter which page your rankings show up on. (If I'm understanding your question correctly!)

I'll further what Highland and Alex Chan are telling you. If you are using Apache (Linux) then you can redirect your old site links using a 301 redirect and .htaccess which is a very powerful tool. Otherwise, if you are using a IIS server, web.config is what you want to use.

A really good resource for .htassess is CSS-Tricks: http://css-tricks.com/snippets/htaccess/301-redirects/

I use the ranking tool quite a bit (It's well worth being a pro member).

What is nice about the MOZ tool is that it checks the keyword phrases you are promoting and shows you your rank, no matter which page your rankings show up on. (If I'm understanding your question correctly!)