If you want a pure vanilla Android without any topping plus the first to get any updates then Nexus is for you.
If you like flashing tons of custom roms and also like to wait for updates (ICS + Touchwiz) then it's SGSII for you.
But certain features in Nexus wont be available to SGSII altough it using the same firmware (ICS)

Yup..there gonna be much fewer custom roms for Nexus than to Galaxy S series. Most developers buy Nexus to port its UI into other phones but seldom built custom roms for Nexus itself. :-)

Its not really the processor here that is the issue. It is whether the developers are willing to put in their effort in another device or not. And they have already put in a lot of effort into the non-G model. However, you may see developers port RAZR or Galaxy Nexus ROMs into the 19100G. But again, whether developers want to support it ultimately is another question.

The thing is, I see a lot of Touchwiz based custom roms on the SGS (i9000) line each time I lurk around xda. Real AOSP based roms, not that many...... And I think we all agree that MIUI and CM are one of the best custom ROMs out there. And they support all the Nexus. As for the i9100, I think its (almost) impossible to find a NEW set in the market. So, that just nails down to the 2nd hang i9100, Note (Note and i9100 use similar hardware, shouldn't be too difficult to port; but better play wait and see approach if ur getting the Note just for custom roms) and the Galaxy Nexus.

EDIT: Take note, currently I do not use any Android phones, I used to use a Galaxy Ace, now using a Nokia phone.

I have the same phones in mind, but I thought about it.
Note way to huge, i saw somebody using it in a restaurant. I don't like the idea of that being a phone.
i9100G - Everyone owns a Galaxy S2, so you don't really stand out from the crowd, and like we know, not many custom toms. Also, the screen resolution is low if you compare with the Galaxy Nexus or Note.
Nexus - Balance of size, has ergonomics so it doesn't feel too huge in your hand, not much bigger than the S2, so it can still easily slip into your pocket, first to receive updates, developers are going to use this device. Pricing hopefully will be bellow the Note's MSRP. The Nexus S still costs a lot if you buy without a contract from Maxis. However, no external storage, luckily for me, I only use about 3GB of stuff for my music. So, shouldn't be an issue.

You have 13 GB usable storage for apps/files. The 13 GB is shared for apps and files unlike previous samsung android phones which had internal storage.

I got my non-G white SGSII less than a week ago. You had to buy AP set if you're really interested in non-G version.
Pros : tons of custom roms. Cons : warranty hussle

all your post since beginning, indirectly tell us that your heart most go to first choice: SGSII. I guess not difficult to get non-g spec if you don't mind used set since it's launched not long ago. still dilemma?    among 3?

note is very laggy. i rate it lower than my sgs2

so your question can closed now.
Note too big for you then you should patiently wait for Nexus. end of story 

many choice custom rom appear for certain model and less for nexus, doesn't meant good or bad.
we look for quality but not quantity    for example, if CM & MIUI promise for Nexus. IMO, already enough, forget for the rest of small player. Since you're in Nexus, you should expected how is the response is. else might try SGSII or Note thread.

Like I said, if you worry about warranty this much, then wait.

YES4G is PAYU (pay as you use), I don't use, I don't pay. If I know my roster involves being out of the country most of the time, I don't top up my YES4G. But it's there if I need to. I deal with large fast data connections for my routines, hence YES4G. If you use your devices for simple activities like Twitter or FB, you don't need YES4G.

There is no "app" that gives data usage for YES4G. Yes Life is an app, yes, but it only functions as your account management for your Yes account, or adding a second number for voice call if you choose as such. To provide data connection on Android, you need a Yes Huddle, a MiFi device. Any WiFi capable device can use the Huddle to access its data connection.

Yes. Just choose the huddle as wifi hotspot

u will be impressed with the speed n fee... 30 goes by snap

That was my point...interesting that we, in Malaysia, for once is getting something released earlier than SG.

lol phone 2 years waranty is more than enough. Who uses phone more than 2 years? lol only Aunty Uncle yes...