"I've port-channel between them using 2X1G links for now, can you please tell me if I need to increase interface capacity or 2G uplinks are fine?"
Cannot answer that, with the information provided.
"I'll monitor portchannel , should I look for anything specific besides output drops on portchannel??"
Yes, many things. Perhaps the most important, and so far unmentioned, is how are the user network applications performing? For instance, you mention "phones". VoIP phones have demanding network service requirements. Many of which can only be met by providing sufficient bandwidth.
So, for example, if all 150 (?) VoIP phone were active, all using g.711, they would need about (100 Kbps * 150 phones) 150 Mbps, easily handled by a single gig link, assuming you're using QoS to insure the VoIP phone both obtain this bandwidth and, further, have priority over other traffic (to meet latency and jitter requirements).
Yet, since 150 gig connected users, could in theory, utilize up to 150 gig, even dual 10g uplinks could be insufficient. Only by monitoring network application performance, can you really determine your bandwidth requirements.
Something, though, to keep in mind. Many network applications are "elastic" in their bandwidth requirements. What they might want to use, or could use, can differ greatly with what they actually require.
As in the case of VoIP, with QoS you can manage bandwidth, insuring applications might obtain bandwidth they really need, at the expense of those with more "elastic" requirements.
Lastly, low link utilization doesn't always mean you have sufficient bandwidth, and conversely, high/full utilization doesn't mean you have insufficient bandwidth. Again, it really depends on network application (for acceptable performance) bandwidth needs. (BTW, my experience has been, using QoS, often allows you to use dramatically "less" bandwidth [because you use it much more efficiently].)
