DSL or T1

DSL is generally FAR less expensive than T1, but part of what you're not getting for the cheaper price is reliability. DSL has no service guarantees, though personally my DSL service doesn't go down very often, and rarely for extended periods (service reliability is almost as good as the phone lines it runs on). What the situation is for your location I have no way of knowing. T1 service normally will have service guarantees, and a higher level of support.

Whether a symmetrical connection is important depends on what kind of usage you have. If your users are typical folks doing web browsing and typical emailing, symmetrical probably isn't that crucial. If they're doing a lot of sending of large files (as email attachments, FTP uploads, or P2P filesharing), or if you're running your office phone system with VOIP, the extra uplink speed could be significant.

If you're consistently having 50-75 users on simultaneously, I doubt that either of these is going to be enough. If they're doing most of their computer usage internally and just occasionally going out to the Internet, then it might be OK. However, I'd seriously consider getting two DSL lines and buying a Dual WAN router that can balance the load between the two connections, so you're less likely to have contention problems. Last I knew, two DSL lines were still just a tiny fraction of the cost of a single T1 line.

T1 gives you static bandwidth and high reliability (it will likely be actively monitored).

DSL is "best effort" and no promises. Also, much cheaper than a T1 as mentioned.
I agree with ElrondCT, with a slight addition (if possible in your area).
Dual DSL lines - if possible, 2 vendors... should 1 go down, you still have the other.
   (Verizon DSL, Local Company DSL)

As long as your business is not totally dependent on the Internet being available, and you are not hosting your own critical web servers or other services, then DSL would be your most cost effective solution.
As everyone else said, you do get what you pay for in the way of service level guarantees, stability and priority service during any outage.

Just be sure that you put a good firewall appliance between the users and the Internet. I suggest Cisco ASA5510 for that many users.

I used the DSL but i hate it because when it goes down and when its speed become very slow the ISP Support tell me "We gave you a DSL line with up to 1Mbps"..that does not mean that I will reach this speed. DSL goes down and shared between you and many customers so you will not gain the speed that you want. You will face the problems when your users try to get their big attachments.

with T1 you will not face these problem. Your speed is fixed and it's not shared and it's for you only. It will not go down and the ISP should tell you when they make maintainance for there lines.

So I hope you to use T1.

Best Luck