Invest more in the natural diamond on all fronts because it will hold its value. The synthetic pink diamond will be pretty and possibly pricey but will not add to the longer-term value of the ring.
Regarding sparkly diamonds: What I mean is that diamonds are crystals (like topazes or garnets or amethysts but made of carbon) and they have internal structural planes which skilled diamond cutters can enhance by cleaving expertly. The better the cut, the more sparkly and thus more valuable, the diamond.
There are specific, known ways to cut diamonds to maximise their potential. The 'easiest' of these, because the technique is constrained by the geometry of a circle, is the round brilliant and the proof that the facets are perfectly matched is that with a loupe 'hearts and arrows' are often visible - google for examples. The expert 'round brilliant' cut is well documented and a 'basic'.
Achieveing this level of mathematical perfection is much more difficult in other shapes/cuts.
Cuts other than 'round brililant' are very complicated. Adding the pointy part of the pear shape will require lots of maths and lots of accuracy. The better cut stone in a non-round brilliant shape will be much more expensive because it is more sparkly but also the non-round shape requires lots of extra effort. Read a little about diamond cuts and you'll see what I mean.
There are other shapes (e.g. princess, Asscher, Leo, emerald, marquis(e), old european, etc). Each of these cuts has pluses and minuses and good cutters can assess a stone and determine which cut will suit it.
The venerable Mr Asscher cut the lumpy, raw Cullinan diamond in to 'Granny's chips' - lots of different shapes. Reputedly, he studied the original, raw lump for over a year before he made a move and reputedly after he made his first cut, he collapsed.
@Anonmousse will confirm that a toi et moi with pear-shaped natural white and pink diamonds would be staggeringly expensive compared to a toi et moi with round brilliants in the H/I + SI range.