Been trading on Macs for several years. Any good java platform will work for day trading. I started on Ameritrade Apex using Streamer Suite and added Interactive Brokers Trader Workstation in 2009. I never tried Think or Swim so I cannot comment on it.
Today, I trade longer term large orders on Ameritrade and day/swing trade on IB. The reasons are explained below.
Tools:
Ameritrade has some really good real-time streaming tools including Dow Jones news (costs $12/month for non-Apex), depth chart, actives, and quotescope. However there are tools which are important to day traders that are windows only. These are Heat Map (shows option conditions hot zones) and Strategy Desk (full technical analysis suite). You will have to use BootCamp, Parallels, or VMWare if you want to use these. Also some tools like screeners and option chains are web based and not really real-time. The results of screeners can be saved as watchlists which can then be used in real-time. It is a real mix but if you like ease of use then Ameritrade is the one to use.
IB's platform is difficult to master because all the tools are fully customizable and they give you multiple ways to trade. They are all streaming java based so none are windows only. The paper account allows simulation of anything you can do on the real account during market hours. They have monthly live recorded WebEx webinars for the tools, apis, and trading topics. Non-customers can attend these. Anyone can also view these at anytime after the event. They are under the Education menu. Start with IB Orientation then TWS Introduction. That should give you a flavor for the Trader Workstation platform. Most of tools use multiple pages in the form of tabs. There is no streaming news. You can directly trade off bid/ask on quote and option chain screens, charts, level1/2, and many more. They have multiple presets for any instrument you trade including attaching bracket and other stops based on your order. You can choose which exchange to directly route your order or let their Smart routing choose for you. You can get in real trouble if your setup is not correct. Once you get it setup the way you want and use it for a while, it is really good. Charts are getting better but do use canned time interval values (1,5,15,30 min, 1,2,4 hr, daily). They do allow full technical analysis to be done on the chart. Screeners are limited to dozens of canned single variable choices though they are working to add new variables. However unlike Ameritrade, screeners can be set to update automatically and they can track like actives, use volume or options conditions, and many more conditions. There are also APIs to allow using Excel or programming your own trading platform
Market Data:
Unlike Ameritrade Apex which includes all market data in the commission prices, IB data is pay as you go on a monthly fee basis. Level I and II are separate. Option and stock Level II are separate. If you want OTC Pink Sheets data, the fee is separate. Each exchange charges a separate fee for its level II and trading floor quotes. The main Level I US NYSE/Nasdaq/OTC stock and options data is $10/month. If you trade enough then the fee is waived.
Commissions:
Unlike Ameritrade Apex about ($10 per stock trade, $10 + .75/option trade), the commissions on IB are per contract with no unlimited max. On stocks, it is about $0.50/100 shares depending on the exchange. Generally if you buy 100 shares, you will be charged $1. Above that it is around 50 cents per 100 shares. Commissions on options are generally 0.75/option each way on CBOE and range widely on other exchanges on each trade but they are generally $1 or less per option. If you scale in and out of trades then this saves a lot of money on commissions. If your order is partially filled then you pay what was filled and can cancel the rest rather than the $10 you would pay per trade on Ameritrade.
