Well, the other option is VDSL2, which will give you 200/100 Mbps asymmetric. But that's like a $250 solution.
If both your routers support 5 GHz WiFi, and the non-modem one supports bridged client mode (or can be updated to DD-WRT or OpenWRT), I'd try that. Use a WiFi app on your phone to scan your apartment and see which 5 GHz channels are in use. Put the bridge on a different channel. Aim the antennas so they're parallel to each other (so for upstairs/downstairs, you'll want the antennas pointed sideways). That should get you at least 100 Mbps, and probably 200-300 Mbps.
If the second router doesn't support bridged client mode, you can get a Ubiquiti Nanostation. Those are about $50 and support bridged client mode (as well as a wireless WAN router mode), and have a directional antenna so interference will be less of a problem. The 2.4 GHz models will only get you about 110 Mbps max, but the 5 GHz models should be able to get you 300 Mbps+. Assuming of course that your modem/router supports 5 GHz. Their drawback is that you only get a single LAN port - you'll need a switch if you wish to share the Ethernet connection with multiple devices.
That said, powerline will not be subject to interference if someone else in your apartment building decides to put a 5 GHz hotspot on the same channel.
You can also run Ethernet cables through heating/ventilation ducts. The "traditional" method is to tie it to a cat and let the cat run through the ducts while the owner calls it from the vent where you want the cable to come out. But the cat lovers will hate you for it.

(Edit: This is a joke. Don't try it for real. The cat will make a really bad stink as it decomposes if it gets stuck in there.)