I'm not from OTT or GAT however I have lived in both and have always worked in Ottawa. This question totally depends on your age and situation.
If you are from TO or southern ON, Ottawa would probably more comfortable and familiar to you. I am from Atlantic Canada and Gatineau is more like home to me scenery, size and attitude of the people. Housing in Aylmer is catching up with Ottawa, but In gatineau (old gatineau) there is a huge gap.
You cannot beat Gatineau anywhere in all of Ontario for outdoor recreation, with 5 ski hills, gatineau park with multiple beaches, and numerous lakes and streams and mountains-bordering the city to the north. Gatineau is also nicer scenery wise, with varied topography whereas ottawa is flat. If you are exclusively english however Aylmer is best.
Bottom line it depends on you-if you have multiple children under 5 and are looking for a lower cost of living gatineau will always be lower. If you are retired or do not have kids ON is more than likely better. If you like to live in a larger city Ottawa is 4 times the size of Gatineau and you'd like it better. However if you like more space and the outdoors Gatineau is better.
I'd also like to summarize some differences and clear up some things
-all new neighbourhoods in gatineau have to have buried utilities
-fit and finish of homes in gatineau is not below ottawa standard levels
-standard lots in gatineau are premium lots in Ottawa 50 ft X 100 is the standard in Gatineau
-in Ottawa you pay for water usage in Gatineau its part of your taxes and is a flat rate an extra bill does not come
-land transfer tax is not unique to quebec ontario has it as well just exempt for first time home buyers
-while car licensing is lower in ottawa when my combined lic and ins for the year was totalled the qc total was just over half of the ottawa rate
-doctors have to have qc and on lic for you not to pay for services...you can find lots of walkin clinics in ottawa with on/qc drs..you only have to pay if they are not qc lic
-you can get your gas when your at work if you work in ottawa-so thats null
-you can go to cheo or any ottawa hospital to have children using you QC Health card
-you can shop for groceries in ottawa if you think they are cheaper there which there is no difference in reality
-beer is cheaper in QC and way more readily available
-private med services in qc are lower because wages are lower
-electricity is much cheaper in gatineau
-embrun, russell and rockland should also be considered as they have real estate price advantages over ottawa as well-while remaining in comfortable ON environs
-the transit switch is not easy you have to switch transit systems in dwntwn ottawa-ottawa transit is better
-bridge traffic is bad
-qc taxes are higher on income but with daycare of one child are null and void