Switching between resources

Are you making sure to switch the port pins appropriately between modes?

Are you resetting the USCI when you switch modes?

I set the reset bit in CTL1 as follows:

void InitI2C()
{
    // I2C
    UCB0CTL1 = UCSWRST;                        // reset the uart
    UCB0CTL0 = UCMST+UCMODE_3+UCSYNC;        // I2C master, 7 bit address
    UCB0CTL1 = UCSSEL_2 + UCSWRST;            // clock source SMCLK
    UCB0BR0 = 12;                            // 100k
    UCB0BR1 = 0;
    UCB0STAT = 0;
    UCB0I2COA = 0;
    UCB0I2CSA = 0xC0;
    UCB0I2CIE = 0x0;
    //UCNACKIE:    Bit 3 Not-acknowledge interrupt enable
    //UCSTPIE:    Bit 2 Stop condition interrupt enable
    //UCSTTIE:    Bit 1 Start condition interrupt enable
    //UCALIE:    Bit 0 Arbitration lost interrupt enable

    P3SEL = 0;
    P3DIR = 0xDD;                            // set unused pins as output
    P3SEL = 0x3E;

    IFG2 &= ~UCB0TXIFG;
    IFG2 &= ~UCB0RXIFG;
    UCB0CTL1 &= ~UCSWRST;                    // remove the reset
    // enable interrupts
    //UCB0I2CIE = UCALIE+UCSTTIE+UCSTPIE+UCNACKIE;
    IE2 |= UCB0RXIE + UCB0TXIE;
}

void InitSPI()
{
    // SPI
      UCB0CTL1 = UCSWRST;                        // reset the uart
    UCB0CTL0 = UCMSB+UCMST+UCMODE_0+UCSYNC+UCCKPH;    // master, 8 bit, MSB first,
    UCB0CTL1 = UCSSEL_2 + UCSWRST;            // clock source SMCLK
    UCB0BR0 = 4;                            // 250k
    UCB0BR1 = 0;
    UCB0STAT = 0;

    P3SEL = 0;
    P3DIR = 0xDD;                            // set unused pins as output
    P3SEL = 0x3E;

    IFG2 &= ~UCB0TXIFG;
    IFG2 &= ~UCB0RXIFG;
    UCB0CTL1 &= ~UCSWRST;                    // remove the reset
    // enable interrupts
    IE2 |= UCB0RXIE + UCB0TXIE;
}

While an SPI slave will ignore all bus activities while not being selected (separate CS signal), the I2C slaves woN't be deaf when you push the SPI data over the line. YOu might accidentally address a slave, or jsut confuse them by apparent start conditions or such.

At least you should do a dummy transfer (start with an unused slave address, then stop) before trying an I2C transfer after an SPI transfer.

Also, keep in mind that the mandatory pullups for I2C have an impact on the signal in SPI mode. I wouldn't try to go with maximum speed.