This article was updated on Feb 12, 2018 and originally published March 9, 2017

An individual retirement account can be used to put away as much as $5,500 each year ($6,500 for people aged 50 and up) to grow tax-free, or tax-deferred. The tax savings can be huge, and those who start early could very well become IRA millionaires by retirement.

But to open an IRA, you'll need to pick a brokerage to serve as the "house" for your account. See how to shop for a broker for your IRA by following along as we compare two popular brokers, TradeStation and Ally Invest (formerly known as TradeKing), on features that are most important to retirement investors.

Commission prices per trade

Commission prices are only going one direction: Down. In February and March 2017, a number of online discount brokers reduced their prices even further so that most trades now start at less than $5.00 each. Here's how TradeStation and Ally Invest compare on commission prices. 

Brokerage

Stocks and ETFs

Stock Options

Mutual Funds

TradeStation

$5.00 per trade (or $0.01 per share, $1 minimum)

$5.00 + $0.50 per contract (or flat rate of $1.00 per contract)

$14.95 per purchase

Ally Invest

$4.95 per trade

$4.95 + $0.65 per contract

$9.95 per purchase

Data sources: Company websites.

Variable pricing makes the comparison appear a little more complex than it actually is. TradeStation's commissions can fall as low as $0.002 per share for extremely high-volume traders who use its unbundled pricing option, but investors can also pay a flat rate of $5.00 per trade, or $0.01 per share. Excluding options trades, Ally Invest's commission schedule is fixed, so the commission won't vary with the order size.

One way to maximize what you get when you open a new account is to shop for special offers for IRAs. The truth is that after considering bonuses and other special offers, the broker that may have a slightly higher commission schedule may actually be less expensive when perks are taken into consideration.

Mutual funds, ETFs, and commission-free choices

TradeStation and Ally Invest get the advantage when it comes to mutual funds, as their standard prices are substantially lower than the standard prices most other brokers charge. However, other brokers offer some funds that are completely free to trade; Ally Invest and TradeStation do not. 

Brokerage No-Load, No-Transaction-Fee Funds (NTF) Commission-Free ETFs
TradeStation None None
Ally Invest None None

Data sources: Company websites.

Minimum deposit requirement for IRAs

TradeStation has a minimum deposit requirement of $500 to open an account. Ally Invest doesn't currently have a minimum deposit requirement to open a new brokerage account, individual retirement accounts included. 

We should caution that it can be advantageous to start with more than the minimum in some cases. For example, some mutual funds have their own minimum investment requirements, and a higher deposit may qualify you for a cash bonus when you open an account. 

Coin jar with a $50 bill inside.

Minimum deposit requirements vary from $0 to the maximum annual IRA contribution. Image source: Getty Images.

Investing in foreign stocks and ADRs

Neither TradeStation or Ally Invest currently offer trading on foreign exchanges, so you'll have to stick with domestic markets like the New York Stock Exchange or NASDAQ. That said, both allow clients to trade American depositary receipts (ADRs), which represent ownership of a foreign company and trade on U.S. exchanges.

Many companies whose products you use every single day like Sony or Restaurant Brands International (parent of Burger King and Tim Horton's) have tickers that trade on U.S. exchanges, and Ally Invest and TradeStation clients can invest in them.

Mobile app reviews

You can check your account and make trades from your mobile phone or tablet from anywhere around the world. Here's how each broker's users and customers rated their mobile trading apps, as of Feb 12, 2018.

Brokerage

Apple App Store

Google Play

TradeStation

3.2 stars

4.3 stars

Ally Invest

4.7 stars

3.4 stars

Data sources: Relevant app stores.

IRA account fees and service charges

While you should always look at any broker's fee schedule before opening an account, there are two fees that are worth some special attention, because they can often be avoided. The first fee is a maintenance fee, or account fee, which you pay each year just to have an account. The second type of fee is an inactivity fee, which you pay when you fail to meet certain criteria. 

TradeStation charges an annual IRA account fee of $35. It also charges an annual inactivity fee of $50. Accounts are deemed inactive when they do not keep an average end of month equity balance of $2,000, or make at least five trades during the twelve-month period following each anniversary of the account's funding date. 

Ally Invest doesn't charge a fee just for having an IRA, nor does it charge an inactivity fee.

TradeStation vs. Ally Invest for traditional or Roth IRAs

Realistically, the best broker for you depends a lot on how you invest. TradeStation may be the better broker for investors who can make full use of its variable pricing schedule to pay fractions of a penny for each share traded, for example. Less active investors may pay less in commissions and fees with Ally Invest, due to the fact it doesn't charge inactivity fees.

To be clear, The Motley Fool does not endorse any particular brokerage, but we can help you find one that's a good fit for you. Visit Fool.com's IRA Center to compare several brokers all on one page, and see if you qualify for any special offers for opening a new account.

Jordan Wathen has no position in any of the stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.