Well, you could buy the cheapest, but how much cheaper is it?
If you buy an exact replacement, it, and all the hoses and electrical connections will hook right up.
The cheaper one may save you $10 in price and cost you $20 in adapters.
If you buy the name brand heater, you know where to get replacement parts. What about the "cheaper" brand?
If you've gotten good service from your Attwood water heater, I would just replace it with the same model.