I would recommend Luke Shaw for you, I too had to decide between Shaw and alaba but I chose Shaw. At first I put him in my under 19s team and in his second season I put him in my First Team. I used him to replace Adriano in my barcelona career, and my plan was to play Jordi Alba as a first choice and Luke Shaw as a second choice. But my plan had to change and I had to start playing Luke Shaw as my first choice as he was playing better that Alba. Alba had a lower average rating and also alba was getting far too many yellow cards. I'm not 100% but I think when I first bought shaw his composure stat was 10 and that is something I wanted to improve. So I trained him on composure and high amount for his season while he was in under 19s. Now his composure is 12, and he is my best left back in the team. I tried to include a screenshot for you to show you some stats for Alba and shaw, but the site wouldn't let me so I will type them to you.
Although my coach says Shaw is only 2 stars, and Alba is 2.5 stars, in my opinion Luke Shaw is still better. Although they are both on 28 games for the season, Shaw has been playing more than him recently (but at the beginning of my season Alba was playing more). Luke shaw's average rating is 7.14, compared to alba's 6.97. Jordi alba wins for pass completion ratio with 80% compared to Luke Shaws 79%. Tackle completion ratio is the same on 87%. Luke shaw wins cross completion ratio on 15% compared to albas 11%. Luke shaw wins header completion ratio on 55% compared to albas 53%. Luke shaw wins for lowest yellow cards with 3, compared to albas 12. I hope you find this post useful, and I would 100% recommend Luke Shaw for you. Just don't forget to improve his composure.