There is a third option, and that is medication. The 40-50% occlusion, under AHA/AAC guidelines, does not require intervental procedures. There are exceptions.
The COURAGE study (google for more info) a scientific study that had the objective to find if there is any difference in longivity among medication, stent, and by-pass for heart patients. The conclusion is there is no difference.
There is a risk (small) with DES (drug eluding stents) for blood clots (thrombus) up to approximately a year or more, but the anti-coangulates (plavix and aspirin) provde good protection against clots. The DES when compared to the bare metal stent fairs better for fewer restenoses.
About 4 years ago, I had an MI and at that time I had a 98% RCA occlusion (stented with a DES) that was stented to provide more blood/oxygen to the distal portion of the heart where there appeared to be heart wall movement impairment, a 72% occlusion of the circumflex (unstented) and 100% LAD artery. The LAD although completely blocked had developed collateral vessels and provides a natural by-pass. I have been on medication for the last 4 years and do not have any symptoms whatsoever. Every day has been a good day...not even a head cold.
If medication can prevent angina (chest pain, etc.) and based on the COURAGE study , there is no reason in my opinion to have an interventional procedure that in itself is a risk...especially a by-pass. There are circumstances that call for intervention such as an acute heart attack, by-pass when a stent is not possible, etc.