You might have a problem whichever database you use. You need to make sure that the first and last values sent by the emonTx are zero power. The graphing functions in emonCMS will draw a straight line between the last value it received before you switched off to the first value it receives after you switch it on. If one or both of those are not zero, it will show a power and energy when there was none.
You can modify the sketch in the emonTx to ensure that it sends a zero power at switch-on, but if you remove the power to the emonTx before it has measured and sent a zero power from the car, you have a problem. There is nothing that the sketch can do about that.
You will use more Feed bytes with the PHPTimeSeries, but only while the emonTx is sending data. So which is best depends on the ratio of on time to off time. More details of the data format for the two databases is in ‘Learn’.
Remember that the emonTx resets when powered up, so the message count and the energy values start again from zero each time. (This is only a problem of course if you use those values.)