Which is better...

By "portable toilet" I presume you mean a porta potti? :h If so, the Reliance brand appears to be a basic hassock style toilet that captures waste in a bag :E so unless you're looking for a really cheap solution pass and get a porta potti that flushes. We had a Dometic porta potti but didn't care for the method of pressurizing the tank with air which allowed for "power flushing". The best we owned was a Thetford 585 electric flush model with a 5 gal waste tank but it's no longer produced, in fact it appears Thetford now offers mostly just bellows flush with most models. If the cost isn't prohibitive and you have the room for it I'd say your best bet would be an electric flush Thetford Curve.