45mins is very tight, and there is little room for even relatively minor delays of 15-20mins, meaning a higher than average chance of being bumped to the next connecting flight (in which case QR will have organised this by the time you arrive).

If you're okay with that possibility of being told upon arriving in DOH that your connection is now, say, 8 hours instead of 45 minutes, and prefer to work on the basis of best case scenario, then you should book the cheaper option.

Otherwise, if you want to lower the chance of unexpected disruptions, you should choose the second best schedule that you're happy with.