1. It's colder - temperate zone - which is refreshing - in Dec-Feb. The biggest hassle is that many medium-older buildings have no heat and you know the price of electricity. Insulation is unheard of. They sell clothing that makes you look like a Yak herder, but ii warm and cheap;
2. Many people seem to target Foreigners as an easy mark, pick-pocket victim. Perhaps it is payback time for years of American bombs;
3. The people appear less friendly, harder to penetrate. Kids less spontaneously friendly than SaiGon. Spontaneous street fights are common;
4. Seems that SaiGon does as much business in one day as Ha Noi does in a week ... possibly a month;
5, Accommodation is cheaper;
6. Food less sweet than south - better for your waistline;
7. Police more strict about traffic infractions. Be aware that chunks of the city are closed - pols living in the area;
8. Ha Noi preserves it's historic buildings, SaiGon knocks them down;
9. Massive areas are subject to seasonal flooding. Getting drowned inside accommodation is not uncommon;
10. Good bus transit system, new overhead rail system being built;
11. Has the boulevard feel of Paris;
12. Much cooler in Jan-Feb but as hot has SGN in the summer;
13. There were, last time I counted, over 56 knock off Sinh Cafe. There is only one real one - now called Sinh Tourist.
14. Most elements of the travel business are riddled with corruption from the pirate taxi's at the airport (use the VietNam Airlines shuttle bus); crooked taxi operators (always get their plate and logo numbers and their driver operator info - and let them see you doing it); never ride a taxi where the meter isn't on the TOP of the dash;
15. Travel agents are required by law to get a bond from VNAT for $15,000 - most don't have one. Tour guides are supposed to wear genuine licences - most don't. Price fixing cartels were perfected in Ha Noi;
16. Travel agents frequently change their names (although not addresses) to get a fresh start in business;
I'm from Canada and Ha Noi is like Ottawa (or Washington, DC) they are political. There are plenty of places to explore and get to know the north better.
My experiences were gained in 14 months of living there in 3 lengthy periods over three years. Most of the taxi drivers are crooks, many merchants know your name (Sucker).
I definitely enjoyed my later stays as I knew the city and the nicer people.
Your pay increase makes it worth it - you can't beat a lazy coffee at Hoan Kiem Lake first thing in the morning. There again, if the job offer is with your present employer, it might be worth finding out why you are so much valuable up there!
The NEW HANOIAN web site < http://newhanoian.xemzi.com/en/ > has a current view of what's happening.
P.S. Saying you prefer Ha Noi to SaiGon will get you a lot of good will!