(FORTUNE Magazine) – NAFTA is in the bag, the Bolsa beckons, and the best buy for many a Gringo investor looks like Telefonos de Mexico. Small wonder: A backlog of 450,000 potential customers for phone service south of the Rio Grande should mean unit growth of 12% annually for the balance of the decade. Phone rates are indexed to inflation at least until 1997. The stock trades at just 9.4 times 1994 earnings, a 30% discount to the Mexican market. Says J.P. Morgan Securities analyst Simon Flannery: ''Telmex gives investors growth, predictability, and one of the best regulatory environments in any emerging market.'' Most brokers tout Telmex American depositary receipts listed on the New York Stock Exchange, which represent 20 shares of the company's Class L stock and trade for $61.63 per ADR. Less well known are its Nasdaq-listed ADRs: They are equal to a single Telmex Class A share and recently sold for $3.09. Quick arithmetic shows that the two ADRs are nearly equal in value, so which should investors buy? Well, the Telmex L shares have only limited voting rights, so partisans of Yanqui-style corporate democracy may feel drawn to those Nasdaq ADRs instead. Don't be: The Mexican holding company Grupo Carso, together with France Telecom and Southwestern Bell, controls the company through yet another class of shares called AA. The Nasdaq ADRs are hardly likely to give you a voice in important decisions. Liquidity is a more significant consideration: A recent day's volume of the Big Board ADRs was about 3.5 million, vs. just 705,000 for the cheapo Nasdaq counterparts. Of what use are the Nasdaq ADRs then? Well, tight-budgeted folk interested in dollar-cost averaging can buy the ADRs in round lots of 100 and avoid the outrageous commissions that odd-lot trading entails. Once you build up a position in the Nasdaq ADRs, you can swap them for the equivalent in NYSE ADRs. One small snag: The depository for the Nasdaq ADR shares -- one of several banks that actually hold the stock that backs the ADRs -- may charge a couple of cents per share for canceling the shares when you switch.