IMO a MEGA is best to start with. Fully Uno pinout and 5V compatible,
but no issues about too few pins for Serial and I2C and SPI and additional ADC and DAC and >30 more digital pins even providing pwm.
And 6MB more RAM, not to forget.
And they cost not a fortune, almost no difference to Unos any more.
then maybe the TO may buy a Mega clone from China for 10 US$ and donate another 10 US$ to arduino.cc?
(about SD cards)There is one important reason why the Arduino Mega is chosen instead of the Arduino Uno (or any Arduino with the ATmega 328 microcontroller). There is only 2 KB of SRAM in the ATmega 328. SD card buffer requires 512 Byts, that's one quarter of the available memory. By putting a few Serial.print functions in the code, it's pretty easy to run out of SRAM in the ATmega 328 before you know it. As SRAM runs out, heap and stack clashes, and your program's behaviour can become strange and inconsistent. As confusing as it can get, it's hard to tell whether it's the 'bug' in the code or short of SRAM, a situation no one wants to go into. On the other hand, the Arduino Mega has 8 KB of SRAM, that's sufficient to most embedded programming, and plenty of room can be left for future improvement / extension.ref.: http://www.hobbyist.co.nz/?q=interface-sd-card-module-to-mega-sensor-shield
...and some start with an Uno, immediatly purchase a Mega, and then work their way up to Due, Yun,.... and maybe yet once in this century: even a TRE! The TRE will not be out until 2020, so forget about it.