Service on both airlines is alike and excellent, Singapore perhaps a little better. Seats on Cathay are both Longer (I'm 6'5) and more comfortable. You also get down duvets and not some used wool blanket that Singapore gives you. Food on Cathay was more plentiful and I found more options as I like all the dim sum etc. Also Singapore has been flying this product for the last 8 years....it seemed dated and old.

Good luck, it is a good decision to have to make.