So the chain that is controlled by 51% of total nodes will win?
That depends on what you mean by "controlled by" and what the creators of Bitcoin XT decide to do.
In other words, if 51% nodes decide to move to XT wallet, the other 49% will also be forced to move to XT?
No.
the fork will in this case happen if and when 90% of the nodes are XT
No.
It is not possible to accurately count how many "nodes" there are. None of this is based on any percentage of nodes.
Right now the creators of Bitcoin XT are stating that they will wait for 90% of the economic power to switch to Bitcoin XT compatible wallets before their Bitcoin XT wallet will accept a larger (and therefore fork inducing) block. It is possible that they could change their minds and modify Bitcoin XT to accept larger blocks sooner, but I feel that is very unlikey.
I believe when they say "90% of the economic power" they are generally talking about 90% of the hash power (and therefore 90% of the blocks in the blockchain), although I think they have talked about getting some of the merchants that handle the largest amount of bitcoins (such as Coinbase, BitPay, and Circle) to switch before increasing the block size as well.
If, for some reason, they decide to allow Bitcoin XT to accept larger blocks when only a little bit more than 50% of the hash power is using Bitcoin XT compatible software, then there will be two blockchains. There would be no "winning" and nobody would be "forced" to switch if they didn't want to. Instead, each blockchain would act like its own cryptocurrency, and you'd have to check with each merchant (or any individual) before you sent them anything to find out which bitcoin they are using. In that situation, many merchants might choose to accept transactions on both blockchains until one chain reaches an economic superiority.
