Comparing two properties

[Mar Now]

I have a question about comparing a property I own to one I have the opportunity to buy.  I want to evaluate if the second is a better deal, and if so, maybe I will sell the first.  

I own a SFH worth $350k with no mortgage. HOA, taxes, ins and property management fees are $450/mo. and rent is $1700. I've had it for 15 years and just haven't revisited selling it.

I am interested in purchasing a multifamily building which needs rehab work.  Purchase price $450k, rehab costs $120k, but expected value $710k.  Rents would total $5200/mo. I would possibly put 20% down, mortgage, tax, ins, mgmt fee = $2500.  (Kind of oversimplified, I know I'm ignoring repairs, vacancy, etc. for now)

So to compare, would I use cap rate?  If so, on what I paid for each  or on the final value?  The first was purchased so long ago that it's initial price doesn't seem relevant anymore. I'm worried I'm just not getting enough rent, that the money could be better used somewhere else.  

Thanks.