distortion or overdrive?

i have a marshall mg50dfx combo amp and i wondered what would be the best pedal i could buy distortion or overdrive wise because i wondered if there would be much difference between using a pedal and just using the amps controls for fuzz.
are the marshall guvnor or jackhammers like to make a good difference?

I like Boss distortion pedals. The orange one is my fav. I also use a Zoom effects unit, so I can tweek the distortion how I like, so it sounds the same from amp to amp. Those two are both good, I find at least.

I use Boss too, and i own both the SD-1 Super Overdrive and the DS-1 Distortion(the orange one) I use the ds1 for metallica or nirvana stuff mostly its a rhythme type of pedal but the sd1 makes a clean sound and is perfect for solos like randy rhoads. Zakk Wylde uses a sd1 and Jimmy Page uses a ds1 so its just a matter of how you like to sound.

There's alot of good OD's and distortion pedals out there, and I've had most of them at one time or another. Sounds like your looking for "fuzz" or distortion. The BOSS DS or HMs will give you basic distortion. The BOSS MT-2 Metal Zone will give you that nasty death metal sound - very similar to the DOD FX86B Death Metal pedal.
The Marshall Jackhammer is a very good pedal also - very versatile as far as tone and drive. This is similar to the Korg 105od pedal which has 2 programmable A/B channels for overdrive and distortion - very versatile and under rated pedal. You may also consider a tube pre-amp, such as an old ADA MP-1 or MP-2. The MP-1 costs about the same (used) as a new pedal (around $100.00). Metallica used the MP-1 on the black album. I personally have an MP-1 in my rig (noisy, but great tone, and I'll defend tubes to the death).
...if you hit the lottery, get a MB Triaxis.

I prefer multiFX pedals or rack units. There are more versatile than any stompoxes...