Petrie has covered a lot of the bases, including the one where the numbers only say so much. I will try to bring some other thoughts to the table.
Part of the question is, how important is the difference in price? These are going to be primarily for use in the car on trips, right? As Petrie, said, lower resistance is always good for saving batteries, especially if your music player isn't all that great on batteries. If it is, this may be less of a factor.
Anyway, my point is that we don't know which set of headphones will sound "better" (a highly subjective term in the case of sound quality).
Based on the designs, I assume that the Sonys will have more sound isolation than the Senns but this isn't 100% guaranteed.
I have experience with the Sony MDR series. I own a set of MDR-V600s which I use for a lot of audio work. They are
Excellent for sound isolation. If they're playing at any decent volume, I can't hear a thing outside the headphones. I've never used Senn headphones but I have heard nothing but good things from them.
My MDRs tend to be a bit bass-heavy, despite the "studio monitor" slogan used on that product line. <_< Not enough to worry about though unless you're trying to mix with these things. I suspect the MDRs you're looking at will have a similar trend. According to the reviews, they do.
I generally find the Sonys quite nice to listen to. They do give a very full sound IF you wear them properly. With mine, it is vital that you have them sitting slightly forward or you lose some of the treble. Again, probably not a huge concern for casual listening but from a mixing perspective, that's very bad.

:
For casual listening, honestly either one will probably do the trick just fine. I would also consider factors like reliability, style and ruggedness. The Senns, it seems, win hands-down for style. The Sonys will be Big suckers which may look a little silly if you walk around with them outside at all. The Senns will look a bit more "normal". Of course, this is entirely personal style/preference.
Let's break this down.
Larger frequency range: Sony (8Hz to 25kHz) versus Senn (18Hz to 18khz).
Winner: Sony. Does it matter? Not at all.

Why? The extra range provided by sony will
1) take so much juice to generate those super-low frequencies that you'll never hear/feel them anyway.
2) not be audible by any normal human.
3) Mp3 tracks DO NOT contain any data in those extreme Sony ranges.
Battery Life:
Winner: Senns.Does it matter? Quite possibly, if you're going on long trips.
Cord length:
8-10ft.
Winner: Senns. Why? "Convenient belt clip for adjusting the cable length when listening on the go." That's why. B) Does it matter? Maybe. How big are your pockets?

:
Style:
Sony: Big, bulky, intimidating
Senns: Sleeker, smoother.
Winner: Your call. I'd say the Senns are more typical for being seen by the public while wearing.Does it matter? Again, your call. If you will wear them exclusively in the car/in private, it probably doesn't matter.
Weight:
Sony: 9oz
Senns: 4.6oz.
Winner: Senns, for comfort.Does it matter? After a long enough ride, it might.
Comfort:
Sony: Comfortable fit with parallel link free adjustable headband and urethane leather ear pads
Senns: Extremely comfortable to wear due to ultra-lightweight design, even for extended listening
What does this mean? Who won?
Who knows?

Sound quality:
Sony: "Dynamic, cinema-quality" *shrugs*
Senns: "High levels and crisp bass for modern rhythm-driven music" *shrugs*
Very subjective. Chances are, those descriptions aren't even very accurate. You don't get "cinema-quality" from $35 headphones.
Cost:
Senns: $21.49
Sony: $34.99
Does it matter? It's up to you.
I'd say the winner for the casual in-car listener is the Sennheiser set. They're cheaper and they appear to be a more practical (and more compact) portable headphone. Again, a lot of this is open to interpretation.