So whats the difference between say.. An airmiles credit card and a 'Credit Builder' credit card? They're both going to build my CR so why not just go for the airmiles one?
I suspect Credit Builder is Capital One? It's just a name - it means nothing. IIRC the 'Credit Builder' product is a card with a crap limit, huge APR and ridiculously lax lending criteria that they'll hand out to anyone - I guess it's to hook in people who think they've got poor credit. They give them a stupid limit of 200 quid, they max it out anyway and dont pay it back and suddenly Capital One are raking it in - people who completely fail at money are the people credit card companies make money from.
ANY card will build your credit rating and provided you meet the lending criteria for the Airmiles card, if thats what you want, then there is zero reason to apply for the Credit Builder card instead.
The Airmiles card suggests you might be planning to travel. If this is the case I'd pick a card which offers zero fee foreign exchange on foreign purchases - you'll save a fortune over using travel agents etc etc to exchange money before going abroad.
The Halifax Clarity card for example charges 0% on foreign purchases and exchanges at the Visa Wholesale rate - guaranteed to be a better deal than you'd get at, say, Thomas Cook, or on any other credit card.
Read the terms and conditions carefully and make sure you know you've got a chance of getting the card before applying, as you dont want to fill your credit file with searches. Many cards require certain salary, for example - make sure you dont apply for these.