Firstly, welcome to DIY
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I've travlled with both Alphabeds (now alpharooms of course) and Winter Sunshine Holidays. Majorca Accommodation is a sub site of Winter Sunshine Holidays. Obviously created to appeal to people looking specifically for certain destinations.
As for security, and please biatch slap me if I'm wrong here mods, not many UK accommodation companies are bonded by ABTA, so you have to look closely at the security they offer.
Both have given me good service in the past, however if security is your main issue then you may be better off with Winter Sunshine Holidays (or Majorca Accommodation whatever, you know what I mean).
WSH operate a trust account service which, if I understand it correctly, means that they dump your money in an account that is protected by some accountant geezers, who will not allow any money to be removed from that account until your date of arrival. (Again, please contact WSH for more info as I might have got it wrapped round my neck).
As far as I can tell Alphabeds (alpharooms) only offer the safe transit of your cashola to their account. This is not just an Alpha thing, I believe it is many online accommodation operators way. There does not appear to be any security equal to what WSH offer to prevent them declaring bankruptcy and running off with everyones money though.
TBH I don't think you could go wrong with either company mate, but if security is your thing, then your probably best emailing both companies and drilling them both on their security.
Cheers
Jazz.