OW vs *A
I will refer to OW and *A only, ST still not same level. Comparing from an Emerald and *G point of view. Based in Asia using RTW product from time to time, transpac, Asia regional and Asia-EU.
OW
1) First class check in and FC lounge access
2) No extra luggage allowance
3) Mileage posting takes extremely long time (using AA and LA accounts), when flying other members it can take up to one month. *A system seems to be better connected.
4) Having 3 tiers and achieving Emerald seems to be more exclusive (than *G)
5) Good coverage in EU/US/Asia-Oceania and South America, low-low Africa
6) Premium classes focus on C. F class seats limited and few members offer an integral F product (maybe because Emerald have access to check in and lounges). In general older F seats than *A
7) Lounge experience nothing special, especially F lounges lower than *A (only Concorde Room up to level)
8) Stronger RTW product with many options and flexibility.
9) Customer service not standardized due to irregular quality among members, IB for instance doesn't fit. If you get stuck in MAD nobody will help, OW means nothing (RTW tkt). LA GAs also don't care about OW elites.
10) No OW upgrades
11) FAs tend to recognize Emeralds and give a better service
12) Too many rules and fine print on mileage accrural, example BA/AA over the pond and many 0% and 25% miles fares (AA and LA are not as bad)
*A
1) Biz class check in and *G only lounge access
2) *G gets extra luggage allowance
3) Fast mileage post in account. Using UA and LH accounts when flying other members average 5 days (or less) to see miles posted.
4) Would like to have a 3rd tier, too many *G members around
5) Excellent coverage in EU/US/Asia, good in Africa, fair in Oceania, low-low in South America
6) Overall F class experience better than OW.
7) Best F lounges, SQ F, LH F/HON, NH F, LX F, among others.
8) Good RTW product, but too basic and restricted.
9) *A system well connected and easy to move from one carrier to other (RTW tkt). Average quality GA higher than OW
10) *A upgrades help burning miles on several carriers
11) *G members are transparent for FAs (again maybe because there are too many)
12) Less fine print on getting miles (I use UA MP which is more transparent)
I general, for me *A offers the best solution as an alliance. Good coverage, strong and consistent FCY products and smooth travel when connecting within its members. Big problem is South America.
OW has interesting tkt products and a couple of very strong members (CX/BA/LA) and Emerald exclusivity.
Conclusion: Since I fly more than 300k (BIS) a year I use both alliances keeping 1K and Emerald (LA) and accessing all benefits. No need to fly ST (in the past enrolled in DL and KE) but honestly difficult to use.
Hope it helps.