I stayed at a different Sonder on Chestnut St farther west a few years ago, and it was fine. It was basically a cross between a corporate apartment and an extended-stay hotel, with somewhat low-quality furnishings and no character.

Also, check whether the price you're seeing includes things like "mandatory cleaning fees." I remember that the Sonder (which my friend booked) did not include those costs in the upfront price (and he apparently didn't pay attention until after the booking was complete ... sigh.) In our case, I remember the all-in price for the Sonder to be about the same as a regular hotel.

Is there a particular reason you want to be right next to the convention center? That's not the loveliest area of town. I'd personally prefer to stay somewhere by Rittenhouse Square, or maybe Reading Terminal Market, and walk to/from the conference. Of course, that assumes you don't have duties that require you to be at the convention center early and/or late.