You'll get even better cell phone reception....My first shot was a pfizer, due to shortage, my second shot is going to be moderna next week. I'll let ya'll know if I make it.
In all seriousness, there are studies that show that mixing vaccines will lead to even greater efficacy. Right now, they're both in the high 90's, but some studies show mixing the vaccine will increase that to about 98%. Both Pfizer and Moderna are quite effective against the delta variant.
Early on, there were concerns about mixing vaccines, but studies have shown that those concerns were unwarranted with Pfizer/Moderna.