The choice is yours, but there are a few things to consider.
First, neither tuner will take the full output of the 80B, unless the load is within reason. For example, you might (?) be able to load the G5RV on 17 or 10, even though the reactance is rather extreme. However, dumping full power into either coupler on either of these bands will likely zap the coupler.
Both are switched LC units, and they just don't have the matching range a decent T match will have. Nonetheless, they have the same limitation when matching loads of about 25 ohms or less, especially highly reactive ones. It is just the nature of matching networks.
You might want to do a search on
http://www.w8ji.com for G5RV so you have a better understanding of the conditions mentioned above.
Oddly, the MFJ mentions that is has an amplifier bypass relay, which they say is an exclusive feature. That's not so, as 2 of the SGC units have had the feature for at least 10 years.
In any case, if it were me, I'd sans the auto coupler, and buy a decent T match.
