Use Gulpin. Why? Here are some reasons:
1. Whismur is very, very weak until it becomes Exploud, which isn't until level 40. It has very poor stats and can't take a hit at all with those 23/23 defenses.
2. Whismur has a horrible movepool, unless you have TMs. Which this game won't give you until far, far later. It also learns no useful moves before exceeding the level cap. Your best moves would be Echoed Voice and...Pound.
3. The only perk Whismur has is that it evolves at level 20 into loudred. But even then it's bulk is only slightly better than Gulpin's, and it still learns no moves at all. Its best move is only Hyper Voice; Boomburst is learned only by Exploud. By then, you're better off using Noivern as your Boomburster, since it is fast, has a bunch of high powered moves to abuse, and is a friggin DRAGON.
4. Gulpin has good bulk, evolves by level 26, and has a bunch of useful moves it can use even after Julia. Acid Spray is a good move because it continually lowers the foes' SpDef harshly, making your consecutive Acid Sprays stronger. It can also use Yawn to put things to sleep or cause switches, Poison Gas to act Poison things, Stockpile to boost both defenses, Amnesia to raise its SpDef, and Encore to force things into one move. It's also going to learn good moves like Sludge Bomb and Gunk Shot eventually.
Also, by the way, Trubbish might be better even than Gulpin. It has slightly less overall bulk, and evolves later, but it gets Toxic Spikes which is huuuge.