Standard 2B ISDN, if you bond both lines together, runs at 128kbps, which is the same speed as low-end DSL.
As they say, the latency is better.
Depending on charges it can be a lot more expensive 'tho (In the UK, ISDN access is charged by the minute. x2 if you bond both B channels!)
Latency vs Bandwidth 101!
Latency/ping is how long it takes for a packet to get from A to B.
Bandwidth is how much crap that packet can carry in one go. (simplified)
e.g. - I could ring you up and read the contents of Wikipedia to you - You'd hear what I'm saying right away, but it would take years for you to get the whole thing. This is low-latency (Good), low-bandwidth (Bad).
OTOH, if I printed the whole thing out and sent it to you via overland snailmail, it'd take weeks to get to you, but once you get it you have the whole thing. That is high-latency (bad), high-bandwidth (good).