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What silverbytes refers to are filtered words. Google may say that they ignore these words but, try searching using both instances. You'll notice that the results are different. In some cases it might be a few positions, in others it could be a few pages, or even deeper. It has been confirmed that proximity plays a role in all of this.
I'd say experiment with different variations where possible. If you have any historical stats, then I'd be looking to see the percentage of searches with and without filtered words.
Argument 1 - Fit more keywords in the title, then you are more likely to come up in a variety of searches, but the title will not read well.
Argument 2 - Title reads better, but less keywords.
Will people be more likely to click on a title that reads well, or do people just click on titles with the right words in?
In you case, keeping the "de" means you are more likely to come higher in the SERP's for "diseño de páginas web". The trouble is do people really type how they speak?
Our site has a similar title, a kind of "widgets in location" title and we do better for searches containing the "in", but I know there are more searches done without the "in", even though that would not be good spoken English. People tend to search in shorthand and often miss out those little words, assuming the results will be the same.
"in" is a very common word and was not included in your search
We've all seen this, but searching with or without the "in" or a similar word will often bring up very different SERPS. I understand that the engines don't index these words, but if the words were not included in the search, why the differences?
The exception is AJ, I've seem some excellent requests, the only thing they seem to omit is a greetng to Jeeves.
Example, "where can I find a shop that sells widgets within a two and a half mile radius of my mum's house"
Okay, not that bad but close enough!
Users will read "Diseño páginas web" but what difference makes? We just want visits..
You do not have to use exact match searches. Perform the experiment I outlined above. Knowing your audience and their search habits will determine how you format your titles.
Adding three characters (the space and de) is not going to have a negative impact on your title relevance unless of course you are at the maximum number of characters that an SE will spider in the <title> element. If that is the case, then a process of elmination is in order. At that time, you may decide to remove filter words to trim down character count.