Unless further elaborated upon, I will for now just consider Vegeta's training under Whis to have been nothing special and basically "mountain training" with a bonus exercise consisting of surviving a blast from sleeping Beerus.
I can accept this as making sense, as the Saiyans mostly just concentrated on increasing their power through their Super Saiyan forms rather than going back to basics(though pre 25th TB we see all the Saiyans training in base and BoG and Super's BoG Arc show it too), because that would take too long.
I think Toriyama has tried going for this "basics is best" approach ever since the Cell Arc in which he had Goku discard the bulky forms and concentrate on basic SS, taking the slow, but ultimately more rewarding approach, rather than the quick fix, that ultimately proves worthless, when taken to its limit.
In the Buu Arc Toriyama does it again, having Gohan discard all Super Saiyan forms, being at his most powerful in base, with the Elder Kaioshin being particularly dismissive towards Super Saiyan.
And now finally in BoG, post God Goku now only focuses on base and Super Saiyan and has discarded SS2 and 3.
Of course the concept is not really explained that well and it has taken this long for Toriyama to start providing us with better explanations for why base is better. The fact, that he isn't limited by a 13 page manga on a weekly schedule has no doubt made him inclined to provide more exposition.
I for one think, as I have made clear a couple of times, that all forms of Super Saiyan are just manifestations of Saiyans' hidden power, hidden power they are capable of achieving in base with enough training. They transform because they are drawing out power, that their body cannot completely handle, resulting in a state that isn't stable and it just gets worse the farther they go with Super Saiyan 3 as the worst, obviously.
The original Super Saiyan level, I think should not be achievable by weak Saiyans. I'd personally have the minimum requirement at Battle Power = 1,000,000, 'cause it's such a nice round number and it also helps justify, why Gohan didn't turn Super, when having his rage boosts against Freeza(when he attacked 3rd form Freeza, he could have a rage boosted power of say 900,000 and then his ki blast being double that, justifying Freeza having trouble with it). In any case as we can see it takes an exceptional Saiyan to achieve Super Saiyan and a popular fan theory is that their near death power-ups being so massive was a result of them breaking past their natural limits and drawing closer to Super Saiyan. I believe that too.
People are going to argue Future SS Trunks vs Future base Gohan, but I have my own head-canon for that too. So Future Gohan has in over 10 years still not been able go past the normal Super Saiyan tier(I view all SS forms as existing within specific tiers, Future Gohan and Future Trunks being the lowest in the initial SS tier). I personally believe that Gohan was just too stupid to realize what lay ahead(SS Grade II/III/FP) and so he just trained in base and SS, but while he barely made any gains in SS, his base form became hella strong and was thus capable of tangling with an inexperience SS(note I don't really care much for multipliers either).
The musclebound Super Saiyan forms I see as a Super Saiyan trying to forcibly draw out his hidden power, but it resulting in an even more unstable form, that the Saiyan can negate after exclusively training the Super Saiyan form, so it can handle that level of power without bulking up(Super Saiyan Full Power). Going back to basics with the Super Saiyan form also resulted in the Saiyans drawing closer to the new realm that lay ahead, which Gohan achieves at the Cell Games(Super Saiyan 2, duh). I basically described that as how Whis explained it to the Oracle Fish, but I believe it's basically the same thing that's going on in both of those scenarios.
Super Saiyan 3 is then the Super Saiyan line taken to its extreme and I feel comfortable saying, that in Toriyama's DB, this is as far as the Super Saiyan line can go(given Vegeta can surpass a SS3, just using SS2, it seems pertinent to question whether or not SS3 would be achievable by normal standards, but I tend to ignore that, as it goes against my head-canon and I can't have that!). From here on, the only way Saiyans can get stronger again is by drawing out all that power in their neglected base states. Gohan is given a shortcut to that power(and for the record, I'm not buying for one second he could go SS anymore, I prefer to look at what happened in BoG as Gohan originally having been written as a SS, as that would be just up Toriyama's alley to nerf Gohan, so others can look good, only for Gohan to end up in "fake" Ultimate form anyways) and if he had trained over the years, maybe he would have eventually achieved SSGSS.
I consider Super Saiyan God to be a shortcut to that realm of power(SSGSS), whereas Super Saiyan God Super Saiyan is the hard way to achieve that power, but perhaps more rewarding as they can transform into it on their own, whereas Goku needed the help of 5 others to do so, ran out of time, luckily managed to remember that realm, but then returning to the state for one last time to negate Beerus' blast. Originally Goku was just going to be using base and SS from now on, as SS at least was supposed to be as good as God form, but then SSGSS happened and Toriyama's oft quoted script had that line and now Super makes it sound like base Goku and Vegeta are terribly weak compared to SS.
So what is the truth now? I wish Toriyama could just explain the whole deal in no uncertain terms, but for now every other material will keep on stating, that SSGSS/SSB is a more powerful form, so that's what we have to go with until something says something significantly differently.
Hope this post gets read at least. It's been a long time, since I have rambled so much
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