Difference between property and portfolio transaction?

Depends - I think capital and tax structure can be main differences. On the capital side, how do you promotes work? Portfolio or property level? How about debt? Secured mortgages by property? Crossed/pooled debt or unsecured/entity level debt? On the tax side - is it an asset or entity sale? 

From a modelling perspective, all of these can drive the differences/complications that you may need to pick up in your model. If you're buying 15 unrelated buildings with their own pieces of debt in one partnership, you can probably use your same acq model and just roll up the CFs. If it's more complicated than that - there may be more that goes into the model... Ultimately you're still modelling debt/equity cash flows and calculating returns. Mechanics may be a little more complicated and returns different but shouldn't be rocket science.

I do know of some groups who primarily focus on portfolio deals - usually people trying to place institutional amount of capital into a quasi-institutional (or non-institutional) space.