Well, Ameyam, its a tough call. Wingboz is right as usual, of course but you are asking a very specific question that I will try to answer.
Hopefully, based on experience, I expect that others will weigh in, especially if I am just full of it!
You also are expressing that your engine choices are limited in Mumbai and trying to understand the tradeoffs between powerplant and weight.
The bottom line is that even the phrase "3D" means different things to different people.
For some, its just something with plenty of thrust for hover, preferably snappy climb FROM hover.
For most, its that plus extreme throws on a light plane.
For others its just a little extra power with the occasional harrier and/or knife edge at low speed.

I am flying a Saito 91 (4 stroke) and am very pleased with my "glow foamy" target. People are amazed to see such a big plane fly so lightly. My dry weight is 6lb 11oz (3kg) which is on the light side from what I read here. At that weight I would rather remove another 3 oz than add for 3D. Light is good for most 3D manouvers- keeps it nice and slow, floaty. I find flat spins difficult but possible at that light weight & ~40 degree elevator throw. Mine never snaps out at that nice light <7lb weight, heaveir would snap out.
If I were to start over, the one thing that I'd change would be the quicker "spool up" of a 2 stroke engine. Sometimes "3D" becomes "oh crap" and you need a quick shot of thrust to save the plane. If you have any powerful little foamies, with ~100Watts/pound of power, you probably are spoiled like me and often will need "instant thrust". A 2 cycle engine will deliver this better. My Saito delivers about 760 watts of power, by the way- its 760W/Hp to convert and that Saito 91 is a ~1hp engine.
Don't trust me, poke around on the Profile Brotherhood (www dot ****************** dot com) site- I think that you will see that many of them prefer 2 stroke for the same reason- quick spoolup and response. A am concerned that a 120 4 stroke, especially a relatively heavy OS 120, may be just too much weight for good 3D, and 3D recovery from errors- less forgiving.
A good 75 size two stroke is good for basic aerobatics, but most are not enough for real 3D from what I read. So if I were to take my experience and everything that I have read here, I would vote for a 90-95 size 2 stroke. IF you do a 75 , do your homework and get a hot one, propped just right.
The heavier, bigger engines are a lot of unnecessary weight for this lightly built airframe. Most of the fuselage is very thin balsa, ~3/32" (~0.5mm thick), doubled in a few key places.
If you have the patience for break-in and tuning of a glow engine, and find the OS hard to get, consider that Supertigre has a 90 for a good price with good power to weight ratio and that (or any decent 2 stroke 90-100) is probably a good, if not ideal match for this plane with a 3D targeted flying style.

Back in the early pages of this thread(first 20 or so) you can find video of someone hovering the snot out of his Phoenix Extra on a 90 2 stroke for 5 minutes less than a meter off of the ground.
Only fast reserve power and a lot of practice can do that! Balls, money and craziness help, too!

PS if it is always really windy where you fly (>15-20 knots), go for the 120- you'll need the weight!