Well I have a trade with my bike shop and the only thing I have left to upgrade on my bike are the wheels.
At this time my wheels are older 27'' and heavy. and I am thinking about putting 700c's on my bike. He will give me a nice set of used 700c's with tires and tubes for my trade.
I have new long pull calipers so I know they will fit.
Am I loosing or gaining anything with getting 700c wheels? or should I try to find newer 27'' wheels?

You're getting better tire selection and maybe better wheels but that depends on what you already have. There's nothing that fundamentally makes 700c better, it's just the new standard.

The other benefit is that you'll get more tire clearance so you could potentially fit a wider tire than you could before if that's something of interest. The only downside I can think of is that the bike might "look funny" with the slightly smaller wheels than it was designed for but I really doubt it would be noticeable.

ya I am riding a 12 speed bike.

depends on what type of wheel/hub you have now?

I'm guessing the frame will have to be re-spaced for a wider hub. I did this on my commuter, and you really can't tell the difference between the two sizes. It's only 4mm radius difference

It's a personal call really. Theoretically the 700's can be lighter than the 27" so there might be a performance advantage but by the same token the frame was designed for 27" wheels and changing the wheel size will affect ground clearance and steering trail. IMHO either way it's splitting hairs.