Then last week the tenants complained that the toilet was not flushing and it was backing up into the bathtub.  

So far my plan was to add a washer and dryer (it has hook ups already), repaint, landscaping, new heaters, new water heater, new flooring, new kitchen cabinets and appliances, maybe add HVAC, new bathtub and vanity.

Bathroom: should we do bath-tub or standing shower?

I also have some boys that helped carry windows, bathtubs, sweep, etc...  

Its hit or miss Fred, it might be cheaper to just have the bathtub area recovered with tile unless you have a affordable painter that could apply the epoxy.I hope this helped @Fred Ramos feel free to PM me if you have any other questions about this project. 

If your duplex is held in an LLC and somebody tries standing on the edge of a wet soapy bathtub to install a shower curtain and falls and breaks something important on their body, they will sue the OWNER, your LLC, not YOU because YOU didn't do anything wrong.

Yearly inspections went smooth until a new (young) inspector started and "failed" the rental due to "dirty drip pans" on the stove, "mold" (mildew) around the tub in one of the bathtubs and a cracked toilet in the second bathroom.  

Shooting from the hip the shower is likely more expensive, there isn't a significant price difference between a shower pan and bath tub, the square footage of tile on the wall is very similar (unless it is a bigger party shower) but to do the shower right you have to install the glass door which will run a few hundred installed.

After our recent visit to the property the bath tub  looked to be worn and needed to be re finished.

At walk through, fan blades were all caked with dirt, cobwebs and dustbunnies abound, hardwood and tile floors hadn't been swept or mopped, countertops not wiped, exterior of kitchen appliances not cleaned, stove top still semi dirty (stove was brand new a year and a half ago but she claims "it wouldn't wipe off"), kitchen sink with soap debris on it, food stains splattered on kitchen backsplash and baseboards, bathtub not cleaned, closet floors/shelves/blinds/windows not wiped down, and my favorite - remains of a carcass of a hawk (feathers and...?)