Zecco:

Price per trade:
$4.50 – Internet based
$19.95 – Broker Assisted

10 free stock trades if you do 25 of more trades per month or if you have $25,000 in your account.
Options: $4.50 + 0.5 per contract

Margin Rates:

$0-$9,999 - 7.75%
$10,000-$24,999 – 7.50%
$25,000-$49,999 – 7.25%
$50,000-$99,999 – 6.75%
$100,000-$249,999 – 6.25%
$250,000-$499,999 – 4.50%
$500,000 + – 4.00% – Call them, it is negotiable.

TradeKing:

Price per trade:
$4.95 (Both internet and broker assisted)

TRADEKING adds $0.01 per share on the entire order for stocks priced less than $1.00. $100 minimum investment per order in OTCBB and Pink Sheet stocks.

Margin Rates:
$0 – $9,999 – 6.50%
$10,000 – $24,999 - 6.50%
$25,000 – $49,999 - 6.50%
$50,000 – $99,999 - 5.50%
$100,000 – $249,999 - 5.50%
$250,000 – $499,999 - 4.50%
$500,000 – $999,999 - 4.50%
$1,000,000 + - 4.50%

In conclusion: If you interact with the customer service, prefer broker assisted trading and prefer a margin account then TradeKing is better.
If you trade penny stocks, want to get 10 free trades and pay cheap commissions then Zecco is better.