well.. you are in one of those "six of one, half dozen of the other" situations.. at least cost wise... the DSL line plus the phone bill you will get (whether you use it or not...) will prolly be a bit more expensive than cable.
you may want to try and work with comcast. depending on where the demarcation point is, they may be required to replace the cabling in the building... sometimes all the way up to where it enters your apt... depends on the building. You may get lucky with this if comcast does say that HSI is available in your area.
as far as speed... cable will always be faster than DSL... how fast? well that depends on alot... speed varies widely for both platforms. DSL range is from roughly 780k to 1.5mb down... cable can run from 1.5mb - 8mb down.
Do not for one moment think that just cuz comcast (or SBC.... or anyone else for that matter) tells you a downstream speed that you will reach that speed.
DSL speeds depend on how far you are from the CO and cable is shared bandwith with everyone on your trunkline, speed will vary depending on time of day.
DSL is more than adequate to run you online games... even if you only got 780k down
i dont know if the landlorn telling you that you can get HSI (and then not...) is enough to get out of a lease.. but who knows! GL to you there!
however, you may be able to force him to upgrade the internal building cabling if comcast wont...