so if your wondering which starter to get ill tell you its not easy but let me tell you the advantage of all starters in platinum and d/p
chimchar fire type a chimp pokemon
chimchar is very interesting pokemon he does not double in size with 2nd evolution but
he is still very useful because your best chances of getting a fire early on and you will want one because otherwise there is ponyta or a houndoom because they have hardly any fire types in platinum and magmorter stuff wont be for a long time and just a little tip if you choose chimchar evolve him into monferno at lvl 14 before doing your first gym battle or get a budew
piplup the water pokemon a penguin pokemon
piplup is a good choice for the first couple gyms and it turns into empoleon one of my favorites but i didnt choose it so anyways you dont necessarily need it because their are other good water types but empoleon is the only water steel type i know and its very strong
turtwig the grass pokemon a leaf turtle pokemon(i think)
turtwig is a very good choice when torterra because its earthquake is very strong and critical plus its the best starter for stability and a good choice
ok so that was my post on starters i know its not that good but i guess its ok for a first