I will try to be very unbiased in my research but I am sure some will creep in. I believe both have received a lot of good reviews, and I have seen/experienced work from both. But I believe Black Lapel has a better bang for your buck. Especially when it comes to customization, they will literally try anything you ask them to do. Black Lapel also has the best customer service I have ever dealt with (and I have dealt with a lot in suits, and other companies alike). Also, if I may say, I own 7 bespoke suits and a bespoke overcoat, and can honestly say Black Lapels fabric and construction are closer to a bespoke quality than any other online M2M I have ever used or seen the results of.
Also Proper Suit, charges more for the same or lesser quality than Black Lapel. In construction Proper Suit charges extra for premier bemberg. Something that a suit company really should never do, and Black Lapel bemberg is probably my favorite thing about the suits as I would say it is just under the bemberg used in my bespoke suits and equal to some. They also charge a lot more for handmade construction which Black Lapel already does for free. And as I said before black lapel will do anything as far as customization you want for free. Which proper suit may or may not charge you for.
I think the nail in the coffin is with proper suit you meet with someone, but as this is a comparison of online M2M I would suggest going to a bespoke tailor shop and ordering a M2M from them. It will probably be around the same price as Proper Suit and will be measured by a true expert who crafts the suits or has been working along side the person who crafts the suits as apposed to someone who just takes your measurements and goes over customizations. Hope this helps, good luck!
I also do not know proper suit's return/alteration/remake process. But Black Lapel will work with you until you are 100% satisfied (and they really mean that) while donating the returned/remade suits to charities for underprivileged kids needing suits for job/college interviews which I think is pretty awesome!