144 hr Transit visa + ShenZhen VOA

I have 4 weeks of vacation, and my intended itinerary is as follows:
YYZ -> CAN (GuangZhou), spend 5 days (144 hr transit visa), high-speed train down to HK, spend 2 weeks in HK, meet up with friends and travel up again to GuangZhou using the ShenZhen VOA for 5 days (they have a Return Home Permit, I do not), high speed train back down again and fly out HKG -> YYZ.

The first visit seems straightforward, I just need to have a train ticket (dated?) ready. The stay in HK also seems fairly safe. The second visit up is suspect: Can I go to GuangZhou with my ShenZhen VOA? If not, what are my options? Is it cheaper to get a visa ahead of time?