Not trying to be a WUM here, but my 2 cents worth on the United/City who is a bigger club debate.
Obviously Utd is bigger than City, no doubt about that. No club will ever overtake its rivals in the space of a few years, even Chelsea for a while won everything yet now with Abramovic holding back the funding, they are below Liverpool in the pecking order I believe. But anyway, consider this:
City have a young squad playing some attractive winning football under the best manager in the game right now. They are on an upwards trajectory and may yet create more special memories if they for example have a spectacular CL success. Chelsea had CL success but it was a rather dull final that no one remembers now, I'm talking about some 99/05 level spectacular. That should definitely leave a mark on the younger generations getting into football these days.
The Alexis transfer. Usually you have a big club offering the chance to play attacking football and win, vs a smaller club offering more money. If I were moving to Juve for say 100k a week, a team like Lazio would have to offer me at least 50% for me to consider joining them and missing out on the Juve trophy train. Again keep in mind that by Xmas that City had the title wrapped up, so Alexis definitely traded trophies for money. IIRC he chose United because you guys offered him and his agent silly wages and fees, even sillier than what City were offering. It took ridiculous money to sway him to Utd's side (which is why IMO he is a terrible signing for you, pure mercenary after a big pay day, has shown no discernible passion for the shirt in the games I've seen him play for Utd).
Bigger club? Utd without a doubt. Current momentum/form/in vogue team? City. City may well fade away when their owners start getting bored/broke like Abramovich in recent times (compared to his crazy spending 2003-2006), but it looks like they are trying to put a more stable long term philosophy by hiring Txiki and working on the Academy (we will see if it works or not, but they sure spent a lot of money on it). Who knows where they will be in 10-20 years? I personally hope they crash and burn, competition was tough enough for us anyway when it was just Arsenal and Utd winning the league and we were the big 3. Big 5/6 means less chance for us to win the league.