Base Gokuu has a hump at about the middle of his hair. When Gokuu turns Super Saiyan (or when wind blows through his hair, or when he's hanging down, or when he's lying down, etc), he loses the hump.
That's the only real difference, if any. You could bring up how the spikes are pointed in different directions, but that's minor and we see Toriyama wasn't consistent on what direction Gokuu's hair spikes would point.
Compare Gokuu in 1985:
To Gokuu in 1990:
Spikes aren't pointed in the exact same direction (1985 is straighter and 1990 is more curved) and yet, he looks basically the same. The direction of the spikes doesn't matter. This is why UI Gokuu is dismissed as a recolor despite changing the direction of the spikes.
Minuscule af Famalam. It's really just the hump that makes a difference, if anything. But even then, not really. Case in point, here's Pu'er-Gokuu.
You can see the hairstyle isn't exactly the same because he lacks the hump. Despite that, it's not a huge difference and he's mistaken for the real Gokuu (granted, Oolong-Bulma is mistaken for the real Bulma despite being much fatter and shorter, but Yumcha points out that she didn't look the same as he remembered. Nobody says anything about Pu'er-Gokuu not looking like the real Gokuu despite the slight differences in appearance).
So, it really doesn't matter how Gokuu's hair precisely looks. As long as it's spiky in general, he basically looks the same. As we see many times in the series, base Gokuu's hairstyle resembles his Super Saiyan hair when wind flows through it; if they were completely different hairstyles, then it would be impossible for base Gokuu's hair to ever look like his Super Saiyan hairstyle. The fact they overlap at all proves they're not that different.
These all look about the same and Super Saiyan has always basically been a recolor Fammy Fam!