Super Saiyan as the users above posted, is far superior in terms of its utility. It increases your damage, attack/movement speed, and your hit rate/dodge. The duration is unlimited so long as you don't run out of EP, change channels, or leave the server by either DCing/Logging out. It has virtually no repercussion on your stats while using it, and in PvP it can be useful in either Preliminary/Any match to break out of your opponents stun(although the 1 hour CD makes it a one-time thing in a match).
Giant Namek on the other hand is very, very situational. Because of it's 10 minute duration limit, and 1 hour CD(30 minutes with capped CD), in addition to the attack/movement speed nerf it pales in comparison to SSJ in terms of utility. However, when the situation calls for it Giant Namekian can be superior to SSJ. For example if you're trying to tank a group of powerful mobs, along with their Boss you can transform into Giant Namekian. It doubles your physical/energy defense, increases your health by 1.5x(I may be wrong about that), in addition to the other buffs that SSJ gets(except for attack/movement speed).
And unlike SSJ, Giant Namekian has its own relatively powerful AoE moves that accompany the transformation. "Kami's Destruction" attacks your target creating an aoe that affects mobs that surround your target within a 15 meter radius, and the lesser version of the first move which attacks in a straight column "Kami's frustation". Another purpose Giant Namekian serves is to allow non-tanks to tank, if you're a party full of DPS' and a Healer. Instances where the Poko isn't overpowered, they use Giant Namekian to not only deal greats amount of damage to entire hordes of minions, but also increase their survivability to the point where the bosses are virtually harmless to you.
If you're bored/screwing around on platform, and your gloves/staff are powerful enough you can one shot players with relative ease using Kami's destruction just for the hell of it. You'd be surprised at how satisfying it is to slaughter players using Giant Namekian.