According to the MSDN documentation, there isn't any difference except for the order in which the two are interpreted.
For flexibility and ease of use, the HTTP Server API supports four
different ways to specify hosts. The four host-specifier categories
are listed below in order of precedence:
Strong wildcard (Plus Sign)
When the host element of a UrlPrefix consists of a single plus sign
(+), the UrlPrefix matches all possible host names in the context of
its scheme, port and relativeURI elements, and falls into the strong
wildcard category.
A strong wildcard is useful when an application needs to serve
requests addressed to one or more relativeURIs, regardless of how
those requests arrive on the machine or what site they specify in
their Host headers. Use of a strong wildcard in this situation avoids
the need to specify an exhaustive list of host and/or IP-addresses.
Explicit
An explicit host name such as a fully qualified domain name in the
host element places a UrlPrefix in the explicit category. This kind of
host element is matched directly against the Host headers of incoming
requests.
Explicit host specifications are useful for multi-site applications
such as Web servers that deliver different content depending on the
site to which the request was directed.
IP-bound weak wildcard
When an IP address appears as the host element, then the UrlPrefix
falls into the IP-bound Weak Wildcard category. This kind of UrlPrefix
matches any host name for the specified IP interface with the
specified scheme, port and relativeURI, and that has not already been
matched by a strong-wildcard or explicit UrlPrefix. The IP address
takes one of two forms in the host element:
IPv4 Literal String
An IPv4 literal consists of four dotted decimal numbers, each in the
range 0-255, such as 192.168.0.0.
IPv6 Literal String
An IPv6 literal string is enclosed in square brackets and contains hex
numbers separated by colons; for example: [::1] or
[3ffe:ffff::6ECB:0101].
IP-bound weak-wildcard host specifiers are intended for applications
that vary the content they serve based on the route taken by incoming
requests. Do not rely on IP-bound weak-wildcard host specifiers to
enforce security.
Weak wildcard (asterisk)
When an asterisk (*) appears as the host element, then the UrlPrefix
falls into the weak wildcard category. This kind of UrlPrefix matches
any host name associated with the specified scheme, port and
relativeURI that has not already been matched by a strong-wildcard,
explicit, or IP-bound weak-wildcard UrlPrefix.
This host specification can be used as a default catch-all in some
circumstances, or can be used to specify a large section of URL
namespace without having to use many UrlPrefixes.
