Sennheiser HD448 vs HD518

Are you going to be using them as portables or for home listening?

Any particular genre you're into?

Type of headphone? (Circumaural, on-ear, closed, open, in-ear)

Do you have an amplifier?

Max budget?

There are a few better options at that price range. 

Well the hd 448 cost about $55 and the hd 518 are seen around $100 atm.  Ive owned both the hd 428 and the hd 558 which are both quite similar to those.  The hd 428 is almost identical to the hd 448 but i hear the 448 is a bit more detailed.  So i overall id guess they are a great mids headphones, highs are good but a bit lacking in the bass.  But $55 for the hd 448 is a great deal.  I have the hd 558 which is the next one up in the line of the 5x8 series.  Great soundstage, OPEN so they leak sound, good mids and highs as well, but i also hear it has good impactful bass which the 448s lack.  I would get the hd 518s but they are almost twice as much as the 448s. 

If you're planning to use it everywhere, that eliminates open headphones.

Grados out, Senn HD518 are out. 

I have the HD438, they're perfect closed sub-$100 headphones with a little EQing.

They're a tad bassier than the HD448, but that's about the only major difference.

Never tried the Sennheiser HD518, but judging from the general Sennheiser sound

signature which is very laid back, I don't think you'd want them for rock.

HD448 for rock: http://www.head-fi.org/t/535078/closed-headphones-for-rock

http://www.head-fi.org/t/546471/shure-srh-440-or-sony-mdr-v6-or-sennheiser-hd-448

If isolation is important, you might want to look into IEMs. The only IEM I've ever

really tried was the HF5, which were great. Very balanced and detailed sound signature,

amazing isolation and it's pretty durable.

If you still wanna stick to closed, over ear design, then look up Audio Technica M50, 

Shure 440/840 and HD25 which is a little pricier (on ear design), Sony MDR V6,

and maybe the Ultrasone HFI 580. 

Sony MDR V6 has been compared to Grado's forward mids which are great for rock.

They're full-sized, and the wire might be a little difficult to handle if you plan on travelling

light. Might need an amp to shine.

M50s are on the bassy side, with decent highs. Used to be a very popular headphone

till the hype died down for w.e reason. 

The HD25, according to Bcasey is the greatest headphone he's ever had for rock (fwir).

Even surpasses Alessandros. But it's above your maximum budget. Might need an amp

to shine, if you're using an Ipod.

Personally, for me, the best headphone I've ever tried for rock was the Grado SR225.

Absolutely amazing. 

You need to look at different headphones if you like rock music.   Sennheisers are mostly for laid back slow soothing music.