Hi, that's a pretty straight forward question but the answer isn't that straight forward, but I'll give a broad answer to cover more than just gaming.....
First of all I wouldn't go with BT or Tiscali as both impose very restrictive limits on how much data you're allowed to download per month, there are lots of other ISPs who offer non-capped connections, this may not be an issue for you but as a principle I have never gone with a restricted service as broadband after all has always meant to be unlimited but that just my principles I suppose (not to mention that I download A LOT ;D).
A few advantages of DSL has over cable is...
The upload speed is generally half of the download speed, whereas your 2Mbps cable connection gives you a pretty slow upload ratio of an eighth of your download speed. The better upload speeds of DSL would give you better performance when using Peer-to-Peer file sharing, although if you went with BT or Tiscali you couldn't use P2P much due to their 'capping' regime.
If your a serious gamer you may appreciate the guaranteed 'contention ratio' of the DSL services.
Contention means that the connection from your local exchange to the ISP (the pipe) is shared by other users. It is usually 50:1 - meaning a maximum of 50 people will be sharing a. When all 50 users are maxing out their connection at full speed, the 'latency', ping times (for gamers) and possibly the download speed may effected for the individuals.
Many DSL ISP's have different price packages covering 50:1, 20:1, 10:1 contention ratios and the more dedicated you are to gaming (clan members for instance), the lower contention ratios are desirable for those ultra low ping times.
The cable companies' contention ratios are different as they have many more users sharing a faster non-contended 'pipe', but the connection between your house and the local node is shared between you and your neighbours and there's no guarantee of how many houses may be sharing that connection to the node. So if you live in a particularly built-up area with many cable customers the 'ping' times to your game servers may not be as hot as a DSL connection in the same area.
DSL also doesn't require any extra wiring installed into your house other than a BT phone line, the only hardware you need is a DSL modem and a DSL filter.
Cable requires you to have an extra co-axial cable drilled into your house, although as you've already had that done that won't be a consideration for you.
As a final summary, if you're experiencing bad ping times when gaming and there's nothing your provider can do about it, DSL may be your thing. But I suggest not going with the mainstream DSL ISP, go with a lesser known ISP which usually has cheaper or less restrictive deals.
ISP Review has a good ISP top 10 rated by the customers.
Bulldog DSL are currently doing a good deal but you have to switch your phone line to Bulldog also (£10.50 pm) in return for this though you can get a 4Mbps unlimited connection
in certain areas for £19.50pm for the first 12 months
If you currently aren't having any problems with your current connection (I can see it's Blueyonder from your IP address) stick with them as it's not capped and it's a fast and hopefully problem-free connection.
