Is it the Defy 2 or the Defy 1?
If it is the 2, then he would be better off with the Malvern Star as it has a better component range in the Tiagra groupset. If your brother wants to race, this is a much better groupset to start with.
If it is the Defy 1, then it also has Tiagra. The frames are both aluminium alloys but the Malvern Star is triple butted, whereas I think the Aluxx is only double butted (not sure how many tubes for each frame thoug...) Wheels are much of a muchness - similar spec frpom Mavic and Shimano. I'm not sure if the Malvern Star comes with pedals though...To be honest, at this price range, with alloy frames and the same components, your brother would be better off making his selection after test riding both bikes to see which one feels better.
Also, he should buy from a dealer he feels comfortable with - one who will spend time with him answering questions and supporting his purchase rather than rushing him or being pushy. If your brother gets into the sport he will most likely spend A LOT of money there in the future and having a shop he feels comfortable with and who will look after him better is very important. If he is into the sport, it is likely that he will only keep that bike for a short time before he will be keen to upgrade anyway and then a relationship with the team at reputable bike shop is even more important.
Also remind your brother to budget for all the other inconsequentials - pump, saddle bag, spare tube, shoes, helmet and anything else he might need to get started.
sorry for the long winded reply, hope this helps
Also remind your brother to budget for all the other inconsequentials - pump, saddle bag, spare tube, shoes, helmet and anything else he might need to get started.