There are several discounts offered through Playbill, i.e. $25 off Sweeney Todd tickets. Will I get the same or better deal at the TKTS booth?

If they have tickets that day you will - if they don't you won't. It's likely they'll have something - but you'll have spent a lot of time standing on line - and risked not getting a ticke.

What is your vacation time worth?

TKTS gets tickets after others have been sold. Therefore, all things being equal, you will probably get better tickets from Playbill and, in any event, as nytraveler suggested, you won't waste your time waiting on a long, boring line.

You will always get a better price at the TKTS booth but the tics might not be as good (or might not be there). If you chance waiting til you arrive, you could print off discount page from playbill or elsewhere and take it directly to the theater. That will save the online fee. You should also check broadwaybox.com and see if there are better discounts which there appears to be.
http://broadwaybox.com/shows/sweeney...c_tickets.aspx

Usually, the discount for Broadway shows at TKTS is 50% of the regular price, plus a $3 or $4 (I forget which) service charge per ticket. This is less than what playbill.com and broadwaybox.com discounts are. However, the great advantage to using the website discounts is that you don't have to wait in line! The best way to take advantage of these is to use them at the box office as soon as you get to the city. In that way, you avoid paying the $6-7 per ticket surcharge when ordering on line or by phone.
The tickets at TKTS not necessarily worse than what the discount offers have, since many times the theaters release these "better" unsold tickets to TKTS on the show date.

DO NOT go to the TKTS booth in Times Square, you will be waiting in line FOR HOURS.

DO GO....to the TKTS booth at the South St. Seaport. THere is never a line and the booth opens at 10am so you are in and out with tickets in hand quickly.

This booth is where all the New Yorkers go for tickets.

After that walk up Water ST to Chinatown for some Dim Sum.

Have a great time.

It's still more practical to use the discount offers previously mentioned, even though it'll cost a few more dollars.
PS: You don't always have to wait at the TKTS line in Times Square for hours. Try it sometime around 5 or 6 or 7 and you'll find little or no lines there.