I can share some of my experiences. I have used TWS for over 6 months. At beginning, it does not have VWAP, which is a big problem. Late last year, it includes VWAP, which is awesome. However, TWS has some big issues:
1. Unstable, very heavy CPU and memory consuming. It is written in JAVA, which is no wonder to be slow. I used MAC, it regularly froze during the premarket.
2. Although the data subscription is much cheaper than DAS, by default you can only open 3 market depth trader (level 2, just like Montage). If you want to open more, you need to buy the data boost package. It depends on your commission per month. They have a formula to calculate how many market depth trader you can open by default based on your monthly commission. Currently, I have 7 montages in my PC, if I want to have the same number in TWS, it will cost more than DAS.
3. TWS has hotkey, but it does not have hotkey for selling or covering a certain portion of your position. This is very useful feature to scale out your trade. Besides, TWS does not have hotkey for chart and other feature. Every time I switch from 5 minute to daily, or other timeframe, it's very inconvenient.
4. TWS sometimes has random number in the Times and Sales window. I don't know if it's caused by the same issue as market depth trader, it's really annoying.
5. It does not have price marker (YY high low, Y high low, etc.). This is very bothering to draw level before the market open.
Good thing of TWS compared with DAS:
1. The watchlist is synced. Every time you input a symbol in a watchlist, all other watchlist is updated. In DAS, you have to input one watch list by one.
2. The chart trend line is also synced per symbol. You draw a horizontal line, it will also appear in other chart if you switch to the same symbol. In DAS, you have to save it first, and load other chart. This is also very bothering...
3. In my opinion, TWS chart is prettier than DAS. The UI is also better. DAS does not have good UI, but it's really lightweight, and fast.
4. TWS has a completely different login for DEMO account. You will never make mistake on switching between DEMO and live account. I just switch to DAS less than one month. Just like other people, I also made mistake on the account once. LOL.
5. It has Mac version. To use DAS in MAC, you have to either use Parallel, or Vmware Fusion. I use Vmware fusion. But to be honest, I think day trader should buy a windows computer. I have Windows laptop, but for desktop, for some reason, I only have a iMac.
I think DAS is much better for day trading. It will be perfect if it can automatically sync the trend line among charts and watchlist, and also add daily moving average in the price marker!
BTW, I have used Active Trader Pro from Fidelity. It's a nightmare!!!