My take on placement of each sector ---
Retail - this will have lowest avg. salary among four, this is mainly bcoz this is totally domestic market oriented, margin in retail sector are low ( walmart has 4% margin), don't require very high tech knowledge like IT / Finance, plenty availablity (any grad. can enter this sector unlike IT where you need to be a comp grad.)
Salary will be lowest among four, slow growth in salary over the years even when Modern Retail continues to boom
Banking - Rising sector, salary will be good but slightly below GB, tremendous growth potential as Indian bank expand abroad & MNC Bank expand in India, no doubt this will be a sector to watch for..
Good starting Salary, very good growth over the years, stable sector
Global Business - Lots of action here, chance of going abroad, need to do lots of travelling, gr8 salary mainly bcoz forex / foriegn currency play a role here, however risky to global slowdown, recession,
Gr8 Starting salary, hightest among four, good growth over the years
Service - Sunrise sector, Hospitality along supposed to produce 60million

jobs in india ( if govt. plays it's card right) and healthcare is on a rise mainly bcoz low cost & good quality compared to western countires, healthcare outsourcing is one of are to watch for in next 10 yrs as many corporates plans to open healthcare / Medical cities
Good Salary, very good growth over years, will catch banking & Gb in few years down the line