Love & Co - Design should be nice, as Love & Co and SK Jewellery belongs to the same company. According to SK salesperson, their bands and ring settings are made in Italy. I took a look at SK's design recently and their ring settings are indeed nice and gives a more contemporary feel. I believe Love & Co should be the same. As far as I know, Love & Co's bands and ring settings are made in Italy as well. Price wise, I am not sure about Love & Co, but for SK, if not mistaken, half a carat with setting should be around 13K, for their True Love. Most of the diamonds of True Love are graded by IGI. Their diamonds are shiny, but they are not really perfect Hearts & Arrows. At least not the sample I saw. But to be fair, other diamond shops may also claim to have diamonds with perfect Hearts & Arrows, but unless they can prove it, I will still take that as a marketing gimmick.

DeGem - Their signature is Lazare diamonds. The salesperson was good enough to show me the difference between a Lazare and a normal GIA diamond that is available in his shop. Price wise is around 13K+ for a 0.42ct, F color, VS1 clarity, Ideal Cut. Lazare is a branded diamond from overseas, hence there is a premium. But truth to be told, although the proportion of how the diamond is cut can be quite strict, it doesn't really guarantee it has an excellent cut, unless proven. To be fair, buying a Lazare can be safer than buying a random diamond because of their stricter process in diamond selection, but if a GIA graded diamond is cut with excellent proportion, they can too shine as nice as Lazare, less the premium. Oh, btw, Lazare diamonds are graded by themselves, not an independent lab.

Diamond and Platinum - Nope, didn't went into that shop to enquire. 

So for your question... design wise, I will go for Love & Co. It should be more modern. Brand wise, I will go for Lazare, but should be more expensive.

And if you are clueless about what your girl likes, go for Tiffany & Co. or Cartier. Just top-up budget only bro!   

But if you are adventurous, can go and look for diamonds online also. Overall, my personal experience is it is overall cheaper, but you will have to do more work yourself. Do check it out on my blog: http://myengagementringexperience.blogspot.my/ 

Ya, definitely looks nice. I guess I love the ring setting more though.  It's a pity SK does not do setting for loose diamond.

Anyway, their Group CEO explained here:
Besides that, Poh Kong also has their own non-standard cut, the Hemera diamond, with 101 facets. 

And Jann Paul got their 88 facets, 8 hearts 8 arrows, octagon shape (8 sides) diamond... so 'huat'... check it out here. I think they are called D'Amor series. 

Personally, I will stick to the traditional 57 facets diamond, 58 facets (if include the culet, according to Wikipedia). This is because those materials that I can study online mainly focused on this cut and it is easier for me to shop around. Besides, I am a traditional guy. 

I asked one of the salesperson in a jeweller store before, he mentioned that the extra facets will produce more white light, meaning it sparkles more, but does not mean it will produce the necessary dispersion/fire/rainbow effect.

I also went to ask uncle google earlier, and found out that extra facets will indeed produce more scintillation (white sparkles), but at the cost of reducing the diamond's dispersion (fire/rainbow effect), as well as the diamond's brilliance (total light return). Refer here.

So if you go and take a look at Hemera 101 facets diamonds at Poh Kong, try to move the diamond around, it will sparkle like crazy, but you will notice that it will have lesser rainbow, if compared to a traditional 57-facets. But of course to be fair, if you get a poorly cut 57-facets, then it won't produce anything nice also, and on the contrary, if you get a really good Hemera 101 facets, it may produce fire as well. However, if you compare to a 101 facets diamond to a 57 facets diamond that are cut ideally, then 101 facets will show more scintillation and 57 facets will show more fire, and probably brilliance.

So I will summarize like this:

101 Facets/More Facets
Scintillation - Very Excellent
Dispersion - Good
Brilliance - Good

57 Facets
Scintillation - Excellent
Dispersion - Excellent
Brilliance - Excellent

But ultimately, beauty is in the eyes of the beholder. If the buyer feels that Hemera attracts him/her more, by all means just buy Hemera. And if the buyer feels that the traditional 57-facets him/her more, then just buy the standard traditional facets. 

Personally, my first choice will still be buying the 57-facets, because:

1. It has long proven proportions for optimum scintillation, dispersion, and fire.
2. There are really a lot of choices to choose from. Most diamonds are 57-facets.

Just my 2 cents.