I was actually both

Megadeth - Killing is my business and business is good, this song slaps 1:11 mark is too epic.

Metallica - Seek and Destroy, Song slaps this song really is greatest songs of all time the begining intro instruments bodies 

Megadeth - In my Darkest Hour Repeats 3:26 mark to 3:39 that's how you shred guitar, entire song slaps about god reject the the most honest christian follower to Jesus,. I repeat 3:26 mark too many times like 10 times because the guitar shredding to epic.

Metallica - Phantom Lord , at 2:30 the guitar is from the devil :)

I never picked a side. By the time I became a fan of Metallica (around 2002-2003), I don't think it was a big deal any more. Maybe it was back in the day but yeah... pretty lame.

That said, Metallica at their best absolutely smoke Megadeth. Ride the Lightning alone destroys the Megadeth catalogue, who I think were pretty average until Rust in Peace. They got better when they commercialised their sound - Youthanasia and Cryptic Writings would be my top picks for them.

The sides being picked were until mid 90's it lasted very long I remeber back then most fans were aggresive like Sega vs Nintendo. 

Metallica at their best decimates Mega. Lighting, Master and Justice was a trifecta. Mega is good, but they are not Metallica. Post black album Metallica is garbage.

'Tallica. I'm not as familiar with Megadeth's catalogue as I am with Metallica's but I'm sure they have nothing on the likes of Fade to Black, Creeping Death, Call of Ktulu, Master of Puppets, Welcome Home (Sanitarium), One or something more mainstream like Nothing Else Matters.

@Chrkeller I disagree about post Black Metallica. Not as good as the old stuff, but there's some top quality songs there on pretty much every album (except St Anger, but even some of that ain't half bad). Bleeding Me is easily one of the best songs they've ever done. The Outlaw Torn as well. A lot of great material. The biggest issue is the albums are too long, and some of the songs need trimming down (which is an issue they had dating back to Ride the Lightning). If they cut say 3-4 songs from each Load album and Hardwired, and 1-2 from St Anger and Death Magnetic (along with trimming the fat from certain songs on the last 3), they'd fare better I think.

Everybody is entitled to their opinions.  My brother would agree.  I'm just not a fan of anything post Black album.  I don't like their sound anymore.  

I love both and have all albums by both bands, been listening to them since 1992.
I would go with Metallica, although I also love Megadeth.

Reasons:

- They seem to take much more care about what they release, the vast majority of their songs is good or very good, only a few skippers. Probably more than 90% of the songs in their catalog is really good. Their only bad albums are Reload and St Anger. Lulu does not count. Megadeth keeps releasing stuff every 2 years no matter the quality and a lot of their albums are not very good or straight up bad.
- Metallica is better live because James can still sing pretty well. Mustaine just can't sing anymore, and it's been like this many years before his cancer. Unfortunately it's gone. Also Megadeth keeps playing the same songs over and over again. Metallica changes the setlist a LOT, bringing some old songs to the setlist all the time.
- Metallica's lyrics are so much better, even since Ride The LIghtning. Megadeth used to have some great Lyrics in albums like Youthanasia and Countdown to Extinction but it's been a while that Dave's lyrics are almost cringe.
- On the other hand, I need to say Megadeth is much better instrumentally, and if you enjoy analysing the drums and guitars there is no comparison with Metallica. Also albums like Rust in Peace, Youthanasia, Countdown to Extinction and Peace Sells are on par with anything Metallica has ever done, even Master and Black Album.

Final thoughts:
- Load is such an amazing album. I hated it when it was released and I was 16, today its one of my favourites from them. Great melodies, lyrics and overall sound/production. Reload on the other hand is pretty bad except for a few songs.
- St Anger is crap, but have you lisened to a version people re-recorded on youtube, with proper drums and shortened songs? Its actually good.
- Risk is not a bad album and it is very far from being the worst megadeth album.

- Although I said metallica is better, I find myself listening more to megadeth. Go figure...

I absolutely love Load (except for 2 tracks) and think Death Magnetic is excellent (all tracks are good, but listen to the remaster not the original release) and a proof Metallica fans will never be satisfied.