Ok i have travelled in both air lines from karachi to islamabad..although i have travelled in shaheen 2 times in my life as compared to airblue in which i have travelled once...here is my xperience..
The first time i travelled in shaheen was from islamabad to karachi in 2002.My seat had peanuts and other food items stuck in between of the seats..the air conditioning didnt work..I thought to myself this is the worst aeroplane i have ever been to..A very uncomfortable flight...
I travelled in Airblue while ging from karachi to islamabad back in 2006.I hadnt heard of Airblue before so i guessed it was a new aviation company.I personally thought it was one of the best aeroplanes i ever travelled in..The AC was one of the best and there was also the radio ..though there was no tv which was a bit of a con..but still it was one of the best domestic flights i had taken..
Then in 2009 I,along with my family had to go to islamabad.My mother said we will go in shaheen but i forced her to opt for airblue...she said that there was a 2000 rs difference in the ticket and shaheen was cheaper..she said that shaheen was not the same like it was 7 years ago and that it had improved by a large margin..i reluctantly agreed.I boarded the plane and had a bad feeling beforehand...I was really surprised at the rudeness of the crew..i was moving along and the airhost (a guy) tells me to put my bag in the upper storage space. I dunno what his problem was but when i was putting the baggage in the upper compartment,he tells me in a loud and rude voice: "Zaraa jaldi karienge aap!!!!".I along with others were shocked at how rudely he was treating the travellers...then when i was placing it and trying to fit it in the small compartment ,he tells me to fir it vertically..I try to fit it in then he says:"Yaar isko vertically rakho..vertical means seedha..english nai aati?" I lost my cool there and then screamed at him"This is your job not mine you ass hole".Everyone was looking at us and he himself was surprised that i could answer back.For the rest of the flight,the AC didnt work and i was sitting with a man in shalwar kameez who was sweating and smelly..For the return trip i told my mom that if u wanna go in shaheen then i will go in train..
So i can assure you that its better in a train then shaheen...and about airblue i really dont know if it has changed to better or worse in the past 3 yearsw but i can assure you that my experience 3 years back was wonderful...If you are asking about international flights then singapore airlines or PIA has no comparision 
Thankyou for reading