I agree that the .3 uF is no big deal, too small a cap will roll off the bottom end so in this case bigger is better.
I don't necessarily agree that either will be better than the Solens. I would assume that if you can hear a difference at all, it could possibly be for the worse. Either may be more expensive and either may enjoy a better reputation in the DIY community, but that doesn't mean that either will automatically sound better in this particular amplifier. There are very few absolutes in this hobby and changing the components in a well designed piece of equipment can lead to unexpected results.
I don't necessarily agree that either will be better than the Solens. I would assume that if you can hear a difference at all, it could possibly be for the worse. Either may be more expensive and either may enjoy a better reputation in the DIY community, but that doesn't mean that either will automatically sound better in this particular amplifier. There are very few absolutes in this hobby and changing the components in a well designed piece of equipment can lead to unexpected results.