Earlier this month my wife and I split a week at Sun Palace and Le Blanc, and so here is our comparison. Note that we had not originally intended to stay at Le Blanc but switched that portion of our trip from The Royal after reading all the possible issues with illness.
Our opinion was that Le Blanc is definitely the better resort but that it does not come close to warranting the difference in price.
Le Blanc wins handily on service and resort grounds. Service is excellent at Sun Palace but outstanding+ at Le Blanc. That said, we are quite happy with the level at Sun Palace. Things like having a butler fill your ice bucket or unpack for you are not things we value. Regarding the grounds, the pools/beach area at Le Blanc is great. Each resort has 3 pools, but Le Blanc's create 3 distinct areas, including one in back that gets you sun late in the day, whereas Sun Palace has one moderately sized pool and and two very small ones. Sun Palace pool area is the weak link of the resort. Beach is great at both. So we give Le Blanc huge edges in these categories.
Sun Palace has better rooms. I think that's true even without the balcony issue, but include that you won't have a balcony at Le Blanc except for top tier rooms, and that's a win for Sun Palace for us. We really like a balcony.
On food, if Le Blanc has an edge it's small. Buffets at breakfast/lunch are very similar and great at both. I noticed New York Strip was the top cut of steak at Le Blanc buffet, whereas it was sirloin at Sun Palace. Dinners may be slightly better at Le Blanc although we thought their signature French restaurant was poor. Obviously many like it, so opinions vary. Regarding mixed drinks, these were good at Sun Palace but outstanding at Le Blanc. House wine is poor at both. Beer is unremarkable at both.
In our experience the guests probably overlapped 60% (in other words, if you looked around at both resorts about 60% of the people might have been at either resort). The other 40% was mainly attributable to younger couples at Sun Palace who I assume were mostly on a very rare splurge, probably their honeymoon. I don't think there are many guests splurging in the same way at Le Blanc, given the price point.
OK, so those are our opinions, and obviously many will see things differently. Overall, we noticed the differences but were surprised they weren't bigger given the price difference.