Increase liability insurance Vs. Umbrella Policy

[Manshi M.]

I have two properties (combined value - $1M) in NJ. I have to decide between increasing liability insurance (that comes with home insurance) on both to $1M from current $500K or taking a $1M umbrella policy. Here are the quotes for the coverage - 

1. Increasing liability insurance for both properties - $129/year

2. Taking $1M umbrella policy - $147/year. If I go for umbrella policy, I'll have to increase my auto liability coverage which will cost me an additional $141/year.

In a nutshell, the umbrella policy will cost me ~$175/year more. Are there any advantages of umbrella policy over the liability insurance? 

Also, how many times the property value should my insurance be? Is there a thumb rule here?